One For All

One For All (ワン・フォー・オール) is a transferable Quirk, possessed by its current host Izuku Midoriya, and was used by its previous hosts Nana Shimura and Toshinori Yagi.

History
Long ago, a man with a Quirk that could steal Quirks and give them away, forcibly gave his seemingly Quirkless little brother a Quirk or "Superpower" that allowed him to stockpile power in his body. However, the little brother already had a Quirk with no effect other than that it could be transferred to others. The little brother's Quirk and the Quirk given to him by his older brother merged and became One For All.

The little brother somehow discovered this. He had a strong sense of justice and he tried to defeat All For One, but given the huge difference in their abilities, the brother failed. The little brother decided to entrust the Quirk to future generations of heroes, hoping that they would cultivate One For All and one day, put a stop to All For One. Few knew the truth about One For All. The Quirk was passed down for nine generations, growing stronger with the passage of time. Each time the quirk was passed down, it would become stronger in the next host, thus becoming stronger.

Description
All Might explains that One For All is the union of two Quirks: A Quirk that stockpiles power, and a Quirk that could be transferred to others. This mutation was caused by the Quirk, All For One.

One For All allows the host to access stockpiled power. It has not been shown if the Quirk that stockpiles power simply gains more energy as it is passed on (the first holder could not beat All For One, but a recent holder could, which suggests a power increase) or if it is able to imprint other Quirk abilities as it is passed from user to user.

One For All receives its name from the fact that it can be "inherited" by others; it is 'one' Quirk for 'all' people. Like a torch, it is passed on from person to person. The method of inheritance and transference of One For All is done by the recipient ingesting the current wielder's DNA, as shown with All Might giving Izuku Midoriya his strand of hair to ingest which let the latter inherit One For All.

One For All will not transfer to a new recipient unless its owner wishes for that to happen, thus it cannot be forcibly stolen. For example, the Hero Killer Stain ingested Izuku's blood, but did not inherit One For All as Izuku was not even aware of such condition. However, this does not mean One For All cannot be forcibly passed on.

Also, One For All seems to alter the user's appearance in different ways. For Toshinori Yagi, it significantly enlarges his muscles and overall body mass to make him look like a completely different person. For Izuku Midoriya, usage of 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 One For All: Full Cowl. It is unknown what the usage of One For All looked like for its previous handlers, however.

After One For All is transferred to the successor recipient, the predecessor can still utilize the "leftover embers" of One For All, and thus still access its power. However, these leftover embers will not last for long, and the previous One For All will slowly lose their access over the Quirk, resulting in the predecessor losing One For All completely. According to All For One, using these last embers continuously will rapidly increase the loss of the power. After All Might defeated All For One, the embers of One For All were burned out, and he cannot even maintain his muscle form for more than an instant.

Proper usage of One For All requires intense and rigorous training of the body to prevent the successor from being torn apart by the raw power flowing within their body. However, not every wielder is stressed the same, and some can get used to it rather quickly, as noted by Gran Torino, who went straight to actual combat training for All Might, since the latter did not take long to adapt to his then-new powers.

However, it is unclear what the true nature of One For All is. When Izuku was about to lose his match against Hitoshi Shinso, he suddenly felt the presence of multiple people coursing through his body and activated One For All to break free from Hitoshi's control. Izuku theorizes that since his Quirk is the "sacred torch that is passed down through the ages," those people that he felt were the lingering presence of those who had this power before him. He also mentions seeing someone with the same hairstyle of All Might among those who appeared in front of him, indicating that his presence is imprinted on the Quirk.

With the right amount of stockpiled power, One For All grants its wielder many abilities, the most prominent being immense strength, superior to that of most strength-enhancing Quirks, that allows one to fire blasts of wind through. It also increases the user's other physical attributes, such as speed and toughness.

All For One's Brother
The first user of One For All was the little brother of a villain who had the Quirk, All For One. Though small and sickly, he had a strong sense of justice and did not back down from opposing his brother's evil deeds. One For All was formed from the mutation of the first's innate Quirk, and the Quirk forced upon him. The first attempted to defeat his brother with his newly merged Quirk (now called One For All), but was not strong enough. He then passed One For All down to a successor in the hopes that someone would eventually make it strong enough stop his evil older brother, a cycle that continues to this day.

Nana Shimura
All Might's predecessor and the seventh successor of One For All. Little is known about Nana's proficiency with One For All, other than she was not strong enough to bring down All For One before her flame of One For All was gone. She had mentored a boy named Toshinori Yagi and had transferred One For All to him.

Toshinori Yagi
As world-renowned hero All Might, Toshinori is the 8th owner and a very proficient user of this Quirk. Through out his career, he was regarded as the most powerful hero of all time and remained undefeated. He is capable of controlling his own strength and focusing his power into any extremity he chooses without recoil.

Toshinori is probably the best-known user of the Quirk, if not one of the most talented, as noted by Gran Torino that Toshinori did not take long to adapt to One For All. However, due to an untreatable wound that was inflicted on him by All For One, he could only utilize his power for three hours straight; any longer and it would strain his body. And if forced to employ his Quirk more than he should, he may shorten the time limit for using his Quirk as a repercussion. After transferring the power to Izuku, his ability to use One For All slowly diminished, and after his battle with All For One, Toshinori completely depleted all the remains of One for All from his body. Although he can still transform for an instant, he cannot maintain his muscle form anymore, reverting back very quickly.

Izuku Midoriya
Izuku is chosen by All Might to become the ninth holder of One For All. Initially, he could only unleash powerful displays of strength at the cost of injuring his own body, as for ten months prior the focus had been on getting him to the point of being able to physically hold One For All without blowing off his own limbs. Despite the lack of training or control, One For All's physical backlash was the first step Izuku needed to overcome in mastering this power.

When Izuku manifests One For All's power, his body starts glowing and emitting sparks, with red-colored, vein-like streams coursing throughout his body. He first used One For All's full power to destroy a giant villain bot in a single punch at the cost of breaking his arm and both his legs.

Izuku eventually develops a way to minimize damage to himself by focusing One For All's power into his fingers individually. He takes this a step further when he subconsciously holds back One For All's destructive power when using it on a living person. From this point on, Izuku began learning on how to properly modulate the output of his Quirk more effectively.

By focusing One For All in certain portions of his body, he could limit the Quirk's power to avoid breaking his bones. He showcased this in both encounters with Shoto Todoroki during the U.A. Sports Festival. He was able to focus One For All into his hand to break Shoto's defense during the Cavalry Battle, and also to punch him a few times during their match in the finals. At that point, All Might believed that Izuku was capable of harnessing only five percent of One For All's power without backlash. Izuku's fight with Shoto proved he was okay with injuring himself as long as he believed he could be healed later on by Recovery Girl. After the conclusion battle, Recovery Girl made it clear that she now refused to heal self-inflicted injuries of that nature, demanding Izuku find a new way to utilize his Quirk.

It is only after training with Gran Torino that Izuku discovers a way to use One For All with no repercussions. During their time together, Gran Torino taught Izuku that his movements and smashes were too slow, as he treated One For All more like a special move than a natural ability. While training himself in an alleyway, Izuku tried regulating his Quirk's power into his legs in order 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 that he was using One For All solely in the limbs that required it, thus severely slowing down his overall reaction time. To compensate, Izuku developed One For All: Full Cowl, which takes advantage of the five percent Izuku can safely work with to increase his strength and agility by letting this small amount of One For All to flow throughout his whole body.

Using Full Cowl, Izuku could easily scale the side of buildings, jump large clearings between structures and quickly navigate through cluttered areas. Many of his newfound movements are attributed to watching the way Katsuki Bakugo propels himself using his Explosion Quirk. Izuku could spar close to the speed of Gran Torino and even held his own against Stain in his first real fight. Even so, if Izuku does not remain calm and alert while using his Quirk, he might go over his five percent limit and injure himself.

During the rescue training race, Izuku used his Full Cowl to enhance his speed and agility and was able to outpace his classmates. This display garnered much shock from his classmates and All Might.

In his second fight with Katsuki, Izuku instinctively found out how to wield One For All at eight percent of its full power without repercussions. Izuku can access any percentage of the full power of One For All, however, any higher than his current limit will damage his body as seen in his fight with Overhaul.

While in physical contact with Eri, Izuku was temporarily able to use One For All at 100 percent of its full power with no consequences, due to Eri's Quirk constantly rewinding all of the damage Izuku was doing to his own body.

In his training sessions with All Might around the time of U.A.'s Cultural Festival, Izuku revealed to All Might that he had discovered during the internships he could use 20 percent of One For All for a very short time if in a desperate situation. Any longer than a brief instant would result in harm to his body. Lamenting his "inability" to perform any long-ranged moves in the battle against Overhaul, Izuku was told that he already had the capacity to do so. All Might had planned to wait until his disciple had control of at least 15 percent of One for All to begin teaching him said moves, but seeing that Izuku had learned how to briefly bring out 20 percent of One For All, which was more than enough to manipulate wind pressure, All Might decided to instruct him on how to perform them.

Toshinori's Super Moves
Most of the attacks used by Toshinori are based on the names of places in America. Even the training schedule he made for Izuku was named such.
 * Texas Smash (テキサススマッシュ): Toshinori throws a right punch with enough force that the wind pressure sends most things flying away. This move is first used to save Izuku from the Sludge Villain.
 * Detroit Smash (デトロイトスマッシュ): Toshinori performs that same attack as Texas Smash except this move is with a downward punch instead of a straight punch. The punch is powerful enough to create a shock wave large and powerful enough to clear the weather. This move is first used to save Izuku and Katsuki Bakugo from the Sludge Villain.
 * Missouri Smash (ミズーリースマッシュ): Toshinori runs towards an enemy while keeping his hand flat and hits the enemy on the head while running past them. This move is first used against Trapezius Head Gear.
 * Carolina Smash (カロライナスマッシュ): Toshinori runs towards an enemy while keeping his hands in a cross position, then in a downwards cross chop to the enemies head. This move is first used against Tomura Shigaraki, but it is intercepted by Nomu, who withstood the full force of the attack, seemingly unharmed.
 * New Hampshire Smash (ニューハンプシャースマッシュ): Toshinori blasts himself towards his enemy and smashes his body into the enemy, crushing and injuring the enemy with his great weight. This move is first used to propel himself at Izuku.
 * Oklahoma Smash (オクラホマスマッシュ): Toshinori whirls around while enemies have latched onto him, spinning with enough force that when they are released they are easily thrown through concrete. This move is first used against Nomu however it was named during the Kamino ward incident.
 * United States of Smash (ユナイテッドステーテスオヴスマッシュ): Toshinori's final and strongest move. Using the last remnants of One For All left in him, Toshinori delivers a massive punch with enough force to reverberate shockwaves. This unstoppable attack was used to defeat All For One and marked the end of his hero career.

Izuku's Super Moves

 * Smash: A powerful straight punch that does immense damage. This was first used against the zero point robot during the entrance exam.
 * Detroit Smash (デトロイトスマッシュ): The same as All Might's move, except Izuku sometimes does it in the form of an uppercut, creating an air pressure blast.
 * Delaware Smash (デラウェアスマッシュ): Izuku flicks his finger to create a powerful shock wave. Doing this fractures said finger, limiting the number of times he can use this move without gravely injuring himself to 10 times.
 * One For All: Full Cowl (ワン・フォー・オール フルカウル): This technique allows Izuku to activate 5%, and later 8% of One For All throughout his body, letting the power course and flow through him instead of concentrating it in one location, and negating the need to "turn on" the power every time he needs to use it. Activating this technique gives him enhanced strength, speed, mobility and agility. It also prevents him from breaking his bones when attacking.
 * 5% Detroit Smash (5 パーセント デトロイトスマッシュ): Izuku activates One For All: Full Cowl, then jumps into the air and delivers an enhanced downward punch to the opponent. He has also used this move in the form a straightforward punch.
 * Delaware Smash Air Force (デラウェアスマッシュエアフォース): Izuku uses his upgraded gloves to direct a blast of compressed air against his foe, possessing enough concussive force to stun Gentle while he was in mid-air, giving Izuku an opening to catch up to him. By using all five of his fingers, he can launch up to four Delaware Smashes simultaneously.
 * One For All: Full Cowl - Shoot Style (ワンフォーオールフルカウル シュートスタイル): A fighting style of using One For All that deviates from All Might. Izuku realizing that just imitating All Might's style which has its special moves centered on his fists was not the best style for him and was actually causing more harm to his arms, he developed a new style that centers more on his legs with the help of his friends. Shoot Style concentrates One for All: Full Cowl's effects in his torso and legs rather than his arms, allowing him to put out even more power due to the fact that the human leg muscles are naturally stronger than the human arm muscles, granting him more power and stability.
 * Shoot Style: St. Louis Smash (シュートスタイルセントルイススマッシュ): While using the Shoot Style, Izuku leaps into the air and before swinging around to deliver a roundhouse kick to the opponent's face. Izuku used this move to incapacitate Gentle Criminal, who had been empowered by La Brava's Lover Mode Quirk to match Izuku's use of One for All: Full Cowl at 8% of its power.
 * One For All: Full Cowl - 100% (ワン・フォー・オール フルカウル 100): Izuku activates One For All to its fullest extent throughout in Full Cowl, granting him a tremendous boost in speed and strength that allowed him to easily overwhelm the likes of Chisaki. This is first used against Kai Chisaki to protect Eri. The power increase seemingly gives him fire-like trails coming from his eyes and light colored hair. He can only use it infinitely with no damage done to his body while in physical contact with Eri because her Quirk constantly rewinds his body to a previous state.
 * One For All 20%: A technique that allows Izuku to use 20% of One For All throughout his body, it causes a lot of pain to the point that he feels as if his bones are about to break.
 * Manchester Smash: While using 20% of One For All, Izuku leaps into the air and flips forward to bring down an axe kick upon his opponent. Although Chisaki managed to dodge it, it pulverized the ground thoroughly enough that Chisaki wasn't able to completely retaliate with his Overhaul Quirk.
 * One For All 100%: A technique that allows Izuku to attack using the full power of One For All in one concentrated part of his body. After initial use, the limb breaks instantly. He can still use expended limbs in combat, but it causes scarring once they're completely healed.
 * 100% Detroit Smash (100 パーセント デトロイトスマッシュ): While using 100% of One For All, Izuku strikes his target with a lunging punch.
 * 1,000,000% Delaware Detroit Smash (1000000 パーセント デラウェアデトロイトスマッシュ): Izuku activates 100% of One For All during an adrenaline rush and then strikes his opponent by first using a Delaware Smash and then a powerful Detroit Smash, hence the Delaware Detroit Smash. During the climax of his battle with Muscular, Izuku manages not only to overpower but also to launch his augmented opponent with enough force to create an impact crater in solid rock with the strike alone.

Trivia

 * To date, most of the moves using One for All are named after states and cities in the USA (e.g. Missouri Smash, Delaware Smash). This is consistent with the manga artist'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 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 within their heart: "SMASH!".
 * So far every named user has had a number in their name somehow correlating to what number user they are. The 7th user is Nana which is Japanese for 7. Toshinori Yagi has the kanji for 8 in his name. One of the kanji in Izuku's name can also be read as "kyu".
 * One For All's name is an inversion of All For One, showing the differing philosophies between their wielders. One For All showing "One Quirk for multiple people" (through passing it on) and All for One showing "All Quirks for one person". Additionally, as the "Symbol of Peace," Toshinori has used his quirk for the defence of all.
 * One for All, together with its counterpart, All for One, may be derived from the popular motto of the titular heroes of the novel  by Alexandre Dumas.
 * It is unknown if One For All can be passed back to a previous user if the current user allows it.
 * As the truth behind One for All is only known to a select few, Izuku's Quirk is listed as "Super-Power" in U.A. and other records.
 * Toshinori still has a few side effects left over from One for All such as; if he attempts to activate his (former) Quirk he still goes in muscle form for a short period of time although his super strength is gone for good.
 * According to Gran Torino and Toshinori using One for All at 100% took far less time for Toshinori than most users and even Izuku.
 * In chapter 2 of the manga, Izuku states that One for All is "one of the seven great mysteries of the world". The other six "great mysteries" are unknown.

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