One For All

One For All (ワン・フォー・オール) is the transferable Quirk that is possessed by its ninth and current host, Izuku Midoriya.

In the past, it was originally used by Yoichi Shigaraki, and later, its other previous hosts: two unnamed users, Hikage Shinomori, Daigoro Banjo, En, Nana Shimura, and Toshinori Yagi. It was also briefly used by Katsuki Bakugo.

History
Long ago, All For One, a man with a Quirk that allows him to steal other people's Quirks and transfer them to others, forced a Quirk on his seemingly Quirkless younger brother, Yoichi. This Quirk allowed him to stockpile power within his body. However, unbeknownst to either of them, Yoichi already had a Quirk with no other ability than to transfer it to another person. The transference Quirk and the power stockpiling Quirk merged to form One For All.

After discovering this about his newly developed power, Yoichi tried to defeat All For One due to his strong sense of justice, but given the massive difference in their strength, he failed. He decided to entrust the Quirk to future generations of Heroes, hoping that they would be able to cultivate One For All and one day put a stop to All For One. He passed it on to the Second User, a resistance leader who had freed him from the prison cell where his brother was keeping him.

All For One sought out One For All, and as a result, almost all of the users of One For All fought him and were killed by his hand, but not before they each had the chance to pass on the Quirk to a successor so that it could reach the future and elude the villain. The fourth user, Hikage, believed that he would not be able to defeat All For One and decided to use his "turn" with One For All to seclude himself and train the Quirk to be as strong as possible. However, after eighteen years, his body unexpectedly started to deteriorate. Though he blamed it on an unknown disease at the time, in reality, possessing One For All and his original Quirk had dramatically shortened his lifespan, forcing him to pass it on shortly before dying. All For One attempted to steal One For All twice, first from the fifth user, Daigoro, and then from the sixth, En, but failed both times. This failure was because the Symbol of Evil couldn't muster the hatred to overcome the fragments of the wielders' consciousness.

The seventh user, Nana, was the last holder killed by his hand, while her apprentice and heir to One For All, Toshinori, fought All For One twice, winning both battles but suffering significant injuries in the process. Despite his injuries and the pleading of his friend/sidekick, Sir Nighteye, Toshinori continued fighting for another six years before picking a successor, putting the Symbol of Piece's tenure as One For All's wielder at 40 years.

After much deliberation in choosing a successor and being scheduled to teach at U.A., which would help Toshinori find one, he encounters Izuku, a Quirkless boy just like he was. Impressed with Izuku's drive to become a hero and his selfless spirit, Toshinori chooses him to become the ninth and current wielder.

While the Quirk was initially perceived as primarily a source of tremendous power, things started to change as Izuku became more accustomed to the Quirk, and it reached the "point of singularity". Not only did the consciousnesses of the previous users gain a more physical form within a vestige realm that could communicate with each other and Izuku, when prior they were only echoes seen in dreams, but also the Quirks of the predecessors emerged from One For All's core, allowing Izuku to use them.

This sudden growth led to Toshinori researching the users' past to find any information to aid Izuku. It was here when he discovered the autopsy behind Hikage's death, and while he was unsure about it at the time, Toshinori speculated that he'd been able to continue using One For All for so long because, unlike most of its other wielders, he was Quirkless.

Due to this revelation, according to Yoichi, Izuku might be the last user of One For All, as it can only be handled by someone who was originally Quirkless, considering said population has dramatically decreased through the generations. The Quirk has been passed down through nine generations, growing stronger. Currently, One For All is one of the most powerful Quirks in existence; however, it proves very difficult to master fully.

To steal One For All, All For One decided he needed someone with highly malicious intent, powerful enough to overpower the will of both the wielder and the other consciousnesses within One For All. He groomed Tomura Shigaraki for that purpose and filled him with hatred. Then, All For One transplanted his original Quirk onto Tomura and seized control of the younger villain. Using Tomura's will, All For One attempted to steal One For All from Izuku but was thwarted by Yoichi and Nana. Knowing that Tomura's body was still incomplete, All For One retreated and began to wait for its completion.

Very few were informed of One For All's true nature, fearing what villains would do if they found out that the world's most powerful Quirk could be transferred, as well as discourse among fellow heroes and the general public. Before the start of the series, other than the users, All For One, and his confidant Kyudai Garaki, the only known people who knew the truth were Nana's partner Gran Torino, Toshinori's sidekick Sir Nighteye, U.A. staff Principal Nezu and Recovery Girl, and police detective Naomasa Tsukauchi. After Izuku nearly gave away the secret to Katsuki Bakugo out of guilt, the latter managed to deduce most of the truth, with the rest explained to him by Toshinori.

However, after the Paranormal Liberation War, the name of "One For All" was leaked to the public when people overheard Endeavor mentioning it during his fight with Tomura. Initially unaware of what it meant, people were able to piece together that it is related to All For One due to the similar name.

In the aftermath of the War, Toshinori was confronted by Best Jeanist and Hawks about One For All and how it was connected to Tomura chasing after Izuku during the battle, so Toshinori discloses everything to Endeavor, Hawks, and Best Jeanist. Izuku and Toshinori also proceed to reveal the truth to Izuku's mother, Inko, and Izuku himself writes personalized letters to each of his classmates in Class 1-A about the secret before leaving U.A. High.

At a press conference, Endeavor, along with Hawks and Best Jeanist, deny knowing anything about One For All to protect Toshinori and Izuku, however, the public continued to question the heroes, as more information started to leak out. While unaware of its transferable origins, many civilians have deduced that One For All is an extraordinary power that Izuku possesses, which is why Tomura is targeting him; this is then confirmed to the civilians seeking sanctuary at U.A. High School. Most of the significant Heroes now know the truth, as they use this knowledge in their planning for the Final War to defeat the Villains for good.

Due to the relationship between One For All and All For One, Izuku is given the primary role in directly dealing with Tomura and All For One's vestige, having trained and mastered the Quirk enough to unlock all the Quirks of the users and use their power to achieve an equivalent of 120% of One For All's power. It has become so powerful it helped disrupt All For One's forceful merger with Tomura, allowing him to strike back at his master and attain full use of the All For One Quirk itself. Proclaiming he does not need One For All, Tomura's goals fully diverge from his master as he proceeds to face down Izuku to destroy him and his Quirk for good, aiming to remove the one person capable of halting his quest for global annihilation.

Description
One For All is a transferable Quirk that can be passed on from one user to the next. Its name comes from its nature: being 'one' Quirk for 'all' people. For the Quirk to be transferred, the recipient must ingest a sample of the predecessor's DNA (ex: a strand of hair or a drop of blood).

The origin of One For All is itself a powerful will that refused to submit to All For One. For this reason, One For All cannot be stolen by any means. The Quirk can only be transferred to another person if the current wielder intends to pass it on, as demonstrated when Stain ingested some of Izuku's blood but did not inherit the Quirk. However, according to All Might, while One For All cannot be stolen, it can be forcibly transferred, as the recipient need not agree to inherit the Quirk in the first place. Since One For All can't be forcibly taken, it is one of the only Quirks that can be used in combat against All For One, as any user of All For One would be able to steal most other Quirks.

One For All also allows the user to stockpile an enormous amount of raw power, allowing them to significantly enhance their physical abilities to a superhuman level, resulting in unbelievable strength, speed, agility, and durability.

The user can focus the stockpiled power into a single body part or spread it evenly throughout their body, although focusing the energy puts a tremendous strain on the part of the body where the power is concentrated. The user can also control the percentage of the power they activate. One For All can even grant the user an exceptional strength boost to their given Quirk if they have one.

The core of One For All has grown in strength from being passed from user to user, and the Quirk Factors of all of the previous users have merged into the core. The current user, Izuku Midoriya, can access the Quirks of the previous One For All users. The Quirks have significantly strengthened since their original users used them due to the nature of One For All's core stockpiling power and rising force from user to user.

In addition to the Quirk Factors merging with the core, One For All carries echoes of the consciousnesses of all previous users. One For All has some subconscious realm where the prior users can manifest. It is later revealed that this is because Quirks are engraved with the consciousnesses of their users, so any new user who receives One For All has their consciousness embedded into the subconscious realm. It was initially believed that these "vestiges" can only be embedded into One For All if the user possesses a Quirk; however, Toshinori proved this to be false since he didn't have a Quirk or a Quirk Factor to have his consciousness copied into, Toshinori instead was able to imbue a piece of his consciousness inside of One For All. This embedment allows the fragment conscious to fully interact with the other users and shares with them everything the real Toshinori knows, which is how the other vestiges were able to learn about the life-shortening side-effect of One For All.

The subconscious realm within One For All was originally a barren landscape surrounded by darkness, and vestiges of the previous holders could only share brief visions with one another. However, after Tomura and All For One attempted to steal One For All, they drew the vestiges out by sheer force, stabilizing the traces within and allowing the current and previous users of One For All to communicate freely. The realm took the form of the ruins of a conference room with chairs, one for each holder, though the second and third users initially chose not to sit and stand in the corner with their backs turned. The realm resembles the room where Yoichi was locked away by All For One, where the stockpiling Quirk was forced on him, and where the Second and Third users eventually rescued him, the birthplace of One For All. It also seems to have some connection to One For All's counterpart Quirk, All For One.

After being stabilized, the previous users can now fully interact with Izuku whenever they wish, freely manifesting as spirits to give Izuku advice on their Quirks and how to use them best. It is also suggested that One For All has a phenomenal ability to allow users to sense other predecessors when they had the Quirk, as if they had a telepathic link to the deceased or former users within the subconscious realm, even long after their deaths, such as All Might sensing the other users, as well as himself, within Izuku despite giving up his Quirk. Izuku felt the wary dangers of Tomura being usurped by All For One, and his crying self deep within the psyche and desired to save him in some way, and Nana and the other former users were able to telepathically sense these emotions because they are now synched with him inside of One For All. Furthermore, since One For All and All For One are counterparts to each other, the vestiges of the previous users resonate with the other Quirk and can peer into each other's inner workings, as shown when the earlier users of One For All were able to see All For One within Tomura during their subsequent encounter, and vice versa.

One For All draws power from the cries of help of those in need, as they are like a direct order to the user. It alters the user's appearance differently. For All Might, it significantly enlarges his muscles and overall body mass to make him look like a completely different person, similar to a giant. For Izuku, using One For All causes the empowered part of his body to glow, with vein-like lines crossing through each other. His body also generates crackling, green energy that resembles lightning bolts when he activates "Full Cowl". However, it is still being determined what the usage appearance of One For All looked like for its previous handlers. Occasionally, while drawing on One For All heavily, a visualization will be shown of the power passing between users, represented as glowing sparks.

One For All itself can be duplicated, but since it is an accumulation-type Quirk, the power stored inside cannot be reproduced as shown when Neito Monoma did not gain any substantial increase in strength after copying it.

Weaknesses
When One For All is transferred, the new user's body is not naturally suited to it as they usually would be with a Quirk they were born with (ex: someone who breathes fire doesn't burn their mouth while using their ability). In addition, One For All is a lot to handle; if one's body can't take the power, then one's body could be blown apart. Because of this, the successor must undergo intense and rigorous physical training.

Even after inheriting the Quirk, it takes time for the new user to adjust to One For All and learn how to properly control it. While Izuku trained his body into a suitable vessel for the Quirk, it was rushed to a period of ten months for him to be ready for U.A. High School's Entrance Exam. Inexperienced use of the Quirk can easily lead to self-injury, most notably with Izuku, who could only use it in short blasts of power that would break his limbs. Only when Izuku learned to channel One For All's power evenly throughout his whole body was he able to stop this self-injurious habit and begin honing the Quirk's power as his own. Even so, repeatedly damaging his body led to extreme consequences, such as the ligaments in his arms becoming frailer, to the point where he now risks losing total function should he continue to use One For All recklessly.

After transferring One For All, the previous user is left with only residual power from the Quirk (or "leftover embers"). While they would still be able to utilize the ability to essentially the same degree as before, continuous usage will cause their power output to decrease until the wielder cannot make tangible use of the Quirk. After All Might's final battle against All For One, his ability to sustain his Muscular Form was reduced to seconds, rendering him unable to make any use of his residual power.

Like with all Quirks, the ability to use One For All is strongly tied to the user's physical health and well-being. As such, they will find it harder to handle the Quirk's raw power should they face negative health issues, even if they were strong and physically conditioned enough to inherit it. Examples here include Yoichi, who could not make proper use of One For All due to his weak body and sickly state, despite being the Quirk's original owner. Another example is All Might, who gained permanent, debilitating injuries following his first battle with All For One, limiting his ability to keep his empowered form active to only a few hours a day and thus hindering the use of his Quirk. As mentioned above, this time frame continued to deplete rapidly after passing One For All on to Izuku.

Following the research conducted by All Might, as well as the discourse of the One For All vestiges, it is revealed that One For All can affect the user's lifespan. If a person who already has a Quirk receives One For All, the extra Quirk burdens their body, depleting the user's life force and shortening their lifespan. It is analogous to adding water to an already-full cup, causing it to overflow and break down. The rate at which the person's life decays is unknown, but Hikage, the fourth user, received One For All at twenty-two and died of "old age" eighteen years later, at forty. This weakness can be circumvented if an inheritor does not already have a Quirk since the Quirkless user would be an "empty cup" filled by One For All. However, this weakness has become more of a concern due to the number of people born Quirkless becoming fewer and fewer, as well as One For All continuing to rise in power, thus limiting potential successors who can handle the Quirk safely.

While One For All cannot be stolen by any means, the users do admit that, theoretically, it is possible for All For One to steal One For All, provided he is able to overwhelm the collective will of all its users. Yoichi speculates that the reason why All For One raised Tomura to have such overwhelming rage and hatred is that it would be able to eat away at the willpower of all the users and therefore bypass One For All's immunity to be stolen.

When manifesting themselves in the unconscious realm of One For All, the current user's body is not all there initially. Some parts, such as their mouth, take time to learn how to fully manifest, making it more challenging to communicate with the other vestiges. This weakness was inadvertently removed by All For One when he unsuccessfully attempted to steal One For All from Izuku. Consequently, there are no communication impediments between the One For All users, allowing the past users to converse with Izuku freely.

Assimilated Quirks
The Quirk Factors of each of the previous users have merged with the core of One For All and have since increased in strength due to One For All's nature of stockpiling power. Izuku is the first user capable of accessing the Quirks of the previous users. Because All For One would constantly hunt and steal the most potent Quirks, most of the user's Quirks are rather unexceptional by themselves and do not have enough potential to be raised to "Ultimate Move" level. However, despite this, the Second User has stated that the stockpiled energy of One For All has resulted in his Meta Ability evolving beyond anything it once was and could be devastating if not used carefully.

The Quirks within One For All are:


 * Gearshift: The Quirk wielded by the second user. This Quirk allows the user to alter the speed of anything they touch, allowing them to accelerate the force of a target, either by enhancing the impact force of their own strikes or by causing the target to lose control of their own velocity.
 * Fa Jin: The Quirk wielded by the third user. This Quirk allows the user to build up and store kinetic energy by performing repetitive movement, this energy can then be released as an explosive burst of speed and power.
 * Danger Sense: The Quirk wielded by the fourth user, Hikage Shinomori. This Quirk allows the user to detect nearby threats.
 * Blackwhip: The Quirk wielded by the fifth user, Daigoro Banjo. This Quirk allows the user to produce tendrils of dark energy which are useful for long-range grappling and increasing the user's mobility.
 * Smokescreen: The Quirk wielded by the sixth user, En. This Quirk allows the user to generate thick clouds of purple smoke from their body, which can obscure people's vision.
 * Float: The Quirk wielded by the seventh user, Nana Shimura. This Quirk allows the user to levitate.

Known Users and Usages
One For All has been bestowed to nine different hosts, starting with Yoichi Shigaraki, the younger brother of All For One. Alongside the Quirk, its origins and the story between the two brothers have been passed on.

Because One For All can only be transferred willingly, each wielder has some form of personal connection with their corresponding predecessor and successor. Every host was entrusted with the Quirk and the duty of using it to stop the evil of All For One.

All of the previous users of One For All are aware of Izuku being the ninth holder of the Quirk. When All Might was researching the prior users of One For All, he was unable to find any information on the second and third users due to the multi-faceted nature of One For All and the era back then.

Yoichi Shigaraki
The first user of One For All.

Yoichi Shigaraki was a man of average height but was very sick and malnourished throughout his life. Like his older brother, he has white hair that goes down to his shoulders, making him resemble his future adoptive nephew, Tomura Shigaraki. He also had bright green eyes similar to the future ninth wielder, Izuku Midoriya.

Yoichi was the younger brother of the powerful villain, All For One. He was initially believed not to have a Quirk, but in reality, he had a Quirk with no power other than transferring it to another person. One For All was formed from the fusion of his given Quirk and the capacity stockpiling Quirk All For One had forced upon him.

Though small and sickly, Yoichi had a strong sense of justice and did not back down from opposing his brother's evil deeds. The younger brother attempted to defeat his older brother with his newly-merged Quirk but needed more power.

Yoichi then passed One For All down to his successor in the hopes that someone in the future would eventually stop his evil older brother once and for all, a cycle that continued to the present day. Even long after his death, Yoichi is aware of Izuku being the ninth wielder of One For All due to his consciousness being embedded into the Quirk and showing Izuku the events leading to the creation of the Quirk.

Second User
The second user of One For All.

He is a man with short, salmon-spiky hair, a large scar across the front of his face, which he received during his final battle against All For One, and wears a combat jacket with a large black collar. He is also equipped with gauntlets on his arms, which he uses to attack using projectiles powered up by his Meta Ability: Gearshift.

He was the leader of a resistance movement who fought against All For One and his new world order during the peak of his power. He did everything he could to fight back, including taking the lives of anyone in their way. He initially had plans to kill Yoichi due to his ties to All For One before changing his heart and choosing to save him instead, inadvertently allowing One For All to grow in the first place.

Even on the verge of death by All For One's hands, the Second User demonstrated his steadfast sense of duty and confidence by continuing to fight back till the end and die with a smile on his face against the villain who was crying for an unknown reason.

He initially refuses to accept Izuku as a successor to the Quirk due to his optimistic desires but is eventually convinced by Yoichi. After Izuku tries to use his Meta Ability for the first time, he reminds the young hero about using his Meta Ability as a "last resort" since the power of One For All has changed it beyond what it once was and can be devastating if not used carefully. He then appears to provide Izuku support when he does activate Gearshift, demonstrating its tremendous increase in power and the strain it has on his body, warning him he'll only be able to use it for five minutes.

His proficiency with One For All is unknown; however, his given Meta Ability, Gearshift, initially was only capable of working on small objects, enabling him to shoot projectiles faster than a bullet, which All For One likened to being a "peashooter".

Third User
The third user of One For All.

He is a man with light, cold gray hair tied up in a spiky ponytail, a black bandana covering his forehead, and a combat jacket with small escape tubes coming out his shoulders.

He was part of the resistance fighting against All For One alongside the second user. He was there during the assassination attempt on Yoichi before deciding to rescue him instead.

Like the Second User, he also initially refused to accept Izuku as a successor to the Quirk before coming around to it, thanks to Yoichi.

His proficiency with One For All is unknown, though he likely used it in conjunction with his given Quirk, Fa Jin.

Hikage Shinomori
The fourth user of One For All.

He has a crack on his face near his left eye, running from across his left eye to his left lower lip. This damage came from the strain that One For All put on his body after wielding it for so long. He also has white hair and slender green eyes.

Unlike the other users, Hikage chose not to confront All For One, aware of the enormous power gap between them, and instead spent his time in seclusion, training himself and strengthening One For All for his successor. During the final years of his life, his body started to break down, causing scars. Hikage passed One For All onto Daigoro before dying at the age of forty.

In All Might's notes about the previous One For All users, he scribbled out Hikage's cause of death. It was revealed that All Might had scribbled out the cause of death due to suspicions he had about One For All's side effects, and thanks to this, the users learned the truth that anyone who wields One For All and already possesses a Quirk would have their lifespan shortened due to the burden of power on the body; therefore, only the ones who are Quirkless can wield One For All.

Hikage's proficiency with One For All is unknown, though he likely used it with his given Quirk, Danger Sense. Until Toshinori succeeded this record, Hikage wielded One For All the longest, having wielded it for eighteen years.

Daigoro Banjo
The fifth user of One For All.

Daigoro was formerly known as the Pro Hero, Lariat, described as "super funky". His proficiency with One For All is unknown, though he likely used it in conjunction with his given Quirk, Blackwhip.

All For One attempted to steal One For All from Daigoro in the past but failed.

He gave En One For All by giving him some of his blood in the battle with All For One.

En
The sixth user of One For All.

En was a man with dark hair who wore a high-collared jacket and was pretty quiet due to his attire obscuring his mouth, and his predecessor, Daigoro, introduced him to Izuku.

Little is known about En's proficiency with One For All, other than he was not capable of bringing down All For One and the fact that All For One attempted to steal One For All from him in the past but failed.

He likely used One For All with his given Quirk, Smokescreen.

He gave Nana One For All by giving her some of his hair in a battle with All For One.

Nana Shimura
The seventh user of One For All.

Nana was a very compassionate and noble individual. She is regarded as a truly great and exceptional hero by mighty heroes, such as All Might and Gran Torino.

Little is known about Nana's proficiency with One For All other than she could not bring down All For One. Nana gave One For All to Toshinori sometime before her battle with All For One, which resulted in her death.

She likely used One For All with her given Quirk, Float.

Toshinori Yagi
The eighth user of One For All who was the first of the wielders to be born Quirkless.

Toshinori has wielded One For All longer than any other user, for forty years, while almost all the other users were either killed by All For One or died of old age due to the burden of One For All. The reason Toshinori was able to hold One For All for so long is revealed to be because he was born Quirkless, meaning he could take the burden of the Quirk since he was an empty vessel One For All could fill.

Toshinori, better known as the Pro Hero, All Might, is the best-known user of the Quirk, if not one of the most talented. Gran Torino once stated that Toshinori did not take long to adapt to One For All, having been blessed with a highly resilient body despite being born Quirkless. He could control and focus his power on any extremity he chose without recoil.

Throughout his career, Toshinori was regarded as the most powerful hero and remained undefeated. Intending to be a societal pillar, he put One For All to great use, defeating all sorts of criminals, rescuing countless people from danger, and turning himself into a legend. He demonstrated the true potential of One For All's abilities through these actions, particularly in fighting crime for three days without any rest and resolving pretty much any threat in mere seconds. Most notably, he was the first One For All holder to battle All For One and win, though both suffered severe and untreatable damage that significantly weakened them.

Due to the untreatable wounds that were inflicted on him by All For One in their first calamitous battle, Toshinori's respiratory system and stomach were left in abysmal shape, and from then on, he could only utilize his Quirk for three hours a day; any more and it would place too much strain on his body. If forced to employ One For All outside this limit, the overall time limit for using his Quirk would continue to shorten as a repercussion. It was for this reason that Toshinori opted to become a teacher at U.A. High School so that he could find a successor to whom to pass One For All and so he would have an explanation to the public as to why he was not in the field doing hero work as often.

During his second battle with All For One during the League of Villains' hideout raid, Toshinori depleted a majority of the "remaining embers" of One For All from his body to defeat the villain, essentially leaving him Quirkless once again. Although he can still transform for an instant, he cannot maintain his muscular form anymore as it causes great strain on his body, reverting very quickly. Having retired entirely from heroic activity as a result, Toshinori now dedicates himself to solely assisting Izuku in mastering the power of One For All so that he may become a great hero of his own.

He transferred One For All to Izuku by giving him a strand of his hair.

Izuku Midoriya
The ninth and current user of One For All, and the second user born Quirkless.

Initially, Izuku had little control over the Quirk and could only use One For All at 100% power, grievously injuring his body each time and only in short bursts. While this allowed him to unleash incredible displays of strength, it came at potentially lifelong physical costs, resulting in his visibly scarred, permanently crooked right hand and arm. This bodily damage is because Izuku's training before receiving the Quirk was only the minimum needed for using One For All without his limbs being blown off completely.

Later, Izuku found a way to minimize the damage to himself by focusing One For All's power on just his fingers. While he still broke his fingers, he could strike mighty blows and generate strong air currents.

Izuku likened the power to an egg exploding in a microwave, so to help him control the Quirk, he would envision an egg not exploding despite the pressure. During the U.A. Sports Festival, Toshinori estimated that Izuku could handle 5% of One For All's full power without hurting himself.

During his internship with Gran Torino, Izuku was told by the elderly hero that his movements and attacks were too slow, as he treated One For All more like a special move rather than a natural ability. While training himself, Izuku tried regulating the Quirk's power back and forth into his legs and arms to scale the side of a building. Unfortunately, he discovered that Gran Torino's previous evaluation was correct and failed to do so.

Izuku eventually realized he was using One For All solely in the limbs that required it at that instant, thus severely slowing down his overall reaction time and adaptability. To correct this, Izuku developed One For All: Full Cowl, which takes advantage of the 5% Izuku can safely work with to increase his overall strength and agility by letting this small amount permeate his entire body.

Using Full Cowl, Izuku can quickly scale the side of buildings, jump large clearings between structures, and quickly navigate through cluttered areas. His newfound movements are attributed to watching how Katsuki propels himself using his Explosion Quirk. Izuku could react to the speed of Gran Torino and even hold his own against Stain in his first real fight. Even so, if Izuku does not remain aware and in control while using his Quirk, he might exceed his five percent limit and injure himself.

During the I-Island Incident, Izuku was given a support item by Melissa Shield, known as the Full Gauntlet, which, when equipped, allowed him to execute higher percentages of Smashes without worrying about the backlash to his body, from 30% to 100%; however, the item was destroyed after a couple 100% Smashes against Wolfram.

By the time of his second fight with Katsuki, thanks to all his training, Izuku instinctively found out how to wield One For All at 8% of its total power without repercussions.

During his battle with Overhaul, Izuku discovered that he could use 20% of his full power for a very short time if a desperate situation called for it, as extended usage would cause him to damage himself. While in physical contact with Eri, Izuku was temporarily able to use Full Cowl - 100% with no consequences due to Eri's Rewind Quirk instantly neutralizing any damage Izuku was doing to his body. This form granted him All Might-level strength and speed.

Around the time of the U.A. School Festival, Izuku, lamenting his inability to perform any long-ranged moves in the battle against Overhaul safely, was told by All Might that he already could do so. His mentor had planned to wait until he had firm control of at least 15% of One For All before teaching him, but seeing that Izuku could briefly bring out 20% already, he decided to do so early. After learning the technique and receiving new equipment from Mei Hatsume to help harness the wind created, Izuku was able to put the Ultimate Move to the test in his battle against Gentle Criminal.

During the fifth round of the joint training session between Classes 1-A and 1-B, dark energy tendrils began to fire from Izuku's arm that heavily damaged his surroundings and pulled him along against his will. This state caused him intense pain and made him unable to deactivate One For All. With the efforts of Ochaco Uraraka and Hitoshi Shinso, they were able to stop the rampage of One For All by brainwashing Izuku, sending him into his subconscious, where he met Daigoro, the fifth user of One For All. Daigoro explains that the Quirk Factors of the previous holders had merged with the core of One For All and increased in strength proportionally. Daigoro tells Izuku that there are five other Quirks that he will manifest and that the Quirk he had when he was alive, Blackwhip, had just awoken within him.

After awakening from the dark world within himself and resuming the match, Izuku could consciously activate Blackwhip and control it to a degree. By calming his mind and looking back on the memories of his initial training with All Might for emotional support, Izuku was able to activate and use Blackwhip with his own will briefly but quickly deactivated it due to the pain from his body not being intense enough to handle the enhanced Quirk, and decided that he won't be ready to use it until he fully masters the base form of One For All in the future.

By the time they started their Work-Studies at Endeavor's Agency, Izuku had reached his base limit of One For All at 10-15%. He also starts to learn through Endeavor the idea of parallel processing, allowing him to focus on perfecting one of his abilities at a time before adding another so that he can become more comfortable with both his Air Force attacks and Blackwhip. By the end of their week, Izuku manages to utilize Blackwhip properly, bringing it out to save cars from being destroyed, though he is still only able to use it for just a moment.

Two months later, by the time of the Paranormal Liberation War, Izuku's proficiency with One For All has increased, allowing him to momentarily use 45% without breaking his body. Training with Endeavor and All Might has also increased Izuku's base limit of One For All to 30%, as well as being much more proficient in using Blackwhip naturally. Later during his fight against Tomura, Izuku unlocks Nana's Quirk: Float. Towards the end of the same conflict, he unlocks Hikage's Quirk: Danger Sense.

During a conversation with the vestiges of One For All, Izuku learns of the burden of wielding One For All if a person already possesses a Quirk and how it will shorten the user's lifespan, meaning it cannot be passed onto any average person any longer. Izuku then realizes with the growing power of One For All and the small number of Quirkless individuals in society, and he could potentially be the last wielder of One For All.

By April, Izuku has shown much more proficiency with his additional Quirks and can freely speak with the vestiges while conscious. Some time prior, he unlocked En's Quirk: Smokescreen. His strength and speed have improved to the point where he can fight on even footing with a much more robust Muscular, a villain whom he barely defeated during their previous fight due to the large gap in physical prowess, demonstrating his proficiency in being able to switch between the four currently active Quirks. It's revealed that his body has grown much more substantial, allowing him to withstand better using One For All at 100% and lessening the damage from using full strength in succession, though his body does still sustain some injury.

During a battle against Lady Nagant, Izuku forces out the third user's Quirk, Fa Jin, for the first time, despite no practice or preparation beforehand. Thanks to his parallel processing training, he can utilize multiple Quirks at the same time, granting him a wider variety of options in combat. However, using all of his Quirks at once and switching them on and off in rapid succession is too much for him to handle and will make his body freeze up. Thanks to Fa Jin, he develops a technique known as "Faux 100 Percent," allowing him to temporarily execute 100% worth of One For All without breaking his body.

After unlocking Gearshift and combining it with Fa Jin through the Ultimate Move "Overdrive," Izuku is capable of taking that 100% of power and turning it into what is described as 120%.

Katsuki Bakugo
In a desperate attempt to stop Nine, Izuku decided to transfer the Quirk to Katsuki by swapping blood through the open wounds on their hands, allowing both of them to utilize the immense power granted by the Quirk.

Despite the period of transfer and possession being extremely brief, Katsuki demonstrated excellent proficiency with the Quirk. He could instantly use Full Cowl - 100% without any prior instruction, and his body could handle the sheer power of One For All without being blown apart.

When Katsuki used One For All to enhance his given Quirk, Explosion, it became so powerful that standard blasts could create enormous craters. The temperature of the explosions was also significantly increased, to the point where it could similarly melt through solid rock to lava as Katsuki came into proximity to it.

At the end of his and Izuku's fight against Nine, Katsuki lost One For, and the Quirk remained in Izuku's body. Toshinori initially suspected that the Quirk stayed with Izuku because Katsuki lost consciousness before the transfer's completion, but later theorized that One For All kept itself inside of Izuku due to his willingness to protect others, even if it meant losing the power. This return to Izuku may mean that the transfer only let Katsuki borrow some of the power stockpiling energy from Izuku.

All Might
Most of the attacks used by All Might are based on the names of states and cities in the United States of America. Even the training schedule he made for Izuku was named such.

Izuku Midoriya
Like All Might, most of Izuku's attacks are also based on locations in the United States. Unlike his master and idol, Izuku cannot use the full potential of One For All without damaging his body, so he must use lower percentages of power and employ it in different ways. Thanks to Faux 100 Percent, he can temporarily utilize speeds similar to 100% of One For All's power, close to that of All Might, without injuring himself.

Katsuki Bakugo
During his brief tenure as the tenth user of One For All, transferred to him by Izuku, Katsuki used Ultimate Moves developed by the previous two users before him. Katsuki was able to use 100% of One For All immediately without prior instruction and without grievous injury, however this may be due to the effects of Katsuma Shimano's Cell Activation, which is stated to boost the body's physical condition.

Trivia

 * To date, most of the moves using One For All are named after states and cities in the (e.g. Missouri Smash, Delaware Smash). This naming system is consistent with Horikoshi's desire to model All Might after American comic book heroes, especially in All Might's hero form, ( and ). It is unknown if, like Izuku, the naming theme is passed from teacher to student or if this was unique to All Might, with Izuku's admiration carrying over.
 * According to All Might, to activate One For All, the user must clench their buttocks and scream "SMASH!" within their heart.
 * For Izuku, he visualizes an egg not exploding or a pastry in a microwave getting evenly cooked when activating One For All.
 * So far, except for En, whose family name is currently unknown, every known user's name contains a reference to their number of users.
 * Yoichi's given name has the kanji for "one" (一).
 * Hikage's surname has the kanji for "four" (四).
 * Daigoro's given name has a kanji that is homophonous to "five" (五).
 * Nana's given name is homophonous to "seven" (七).
 * Toshinori's surname has the kanji for "eight" (八).
 * Izuku's given name has a kanji that is homophonous to "nine" (九).
 * One For All's name is an inversion of All For One, showing the differing philosophies between their wielders. One For All shows "One Quirk for All people" (through passing it on from person to person), and All For One shows "All Quirks for One person" (through taking Quirks from others).
 * The names of both One For All and All For One are about the phrase "All for one, one for all", a motto commonly associated with the titular characters of the novel  by Alexandre Dumas.
 * Users of One For All manifest differently to the point where it defies regular classification. All Might transforms into a musclebound giant, which correlates to the Transformation Class. Izuku's veins glow red and could easily place him in the Emitter Class because the power of One For All starts coursing through his veins. It is still being determined if the past and future users will manifest One For All and how it will be appropriately classed.
 * If the current user allows, It is unknown if One For All can be passed back to a previous user.
 * As the truth behind One For All is only known to a select few, Izuku's Quirk is listed as "Super-Power" (超パワー) in U.A. and other records.
 * According to Gran Torino and Toshinori, using One For All at 100% took far less time for Toshinori than Izuku.
 * Izuku states that One For All is "one of the seven great mysteries of the world." The other six "great mysteries" are unknown.
 * One For All, All For One, Life Force, Fa Jin, Gearshift, and the unnamed Luminescence Quirk are the oldest known Quirks in the series.
 * Izuku and Toshinori are the only wielders of One For All who inherited the Quirk as teenagers. The rest of the users had reached adulthood by the time they inherited One For All.
 * Additionally, they are both currently the only inheritors of One For All to have been Quirkless.
 * Of all users of One For All, All Might wield the Quirk for the most prolonged time, forty years. He is followed by the fourth user of One For All, Hikage Shinomori, who wielded the Quirk for eighteen years.
 * The former wielders believe that Izuku will be the last wielder of One For All.
 * One For All as a Quirk is almost a complete inverse of All For One in function and abilities:
 * Both gain multiple Quirks from others but in opposite ways. All For One takes any number of them directly from others against their will. One For All is transferred to others and gains abilities one at a time.
 * All For One can freely give and take Quirks but cannot be transferred without outside assistance. One For All permanently keeps the Quirks it assimilates and can not only be transferred to others but cannot be taken against the user's will, barring extraordinary circumstances.

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