Yoichi Shigaraki/Synopsis

History
Yoichi was born in an era when Quirks began to manifest for the first time in human society. Apparently, he was assumed to be born as Quirkless, while his older brother was born with a very powerful Quirk.

During their early years, the two brothers seemed to have a good relationship, spending time together reading superhero comics. However, when they grew up and society began to collapse because of the rise of Quirks, it was the moment when their different ideologies and mentalities came into conflict. The two had conflicting views regarding the Quirk phenomenon and the anarchy of humanity. All For One saw it as a means to exploit for power whereas Yoichi believed that Quirks should be used to restore order.

When he understood his older brother's intentions, he tried to do what he could to stop him, but having no Quirk and a weak constitution, he could hardly do anything but watch helpless as his older brother committed various atrocities one after the other. All For One stole many Quirks, attaining immense power and influence. Using his incredible might, the man gathered and unified dozens of people under his command. He then manipulated the people he gathered like pawns to further his plans and eventually became Japan's ruler.

All For One tried to show his brother that he was helping people by ridding those who believed themselves plagued and giving strength to the helpless. However, Yoichi saw through All For One's claims, deeming that he is only trying to amass people who owe him into servitude. All For One defended this by stating his Quirk is able to return order to the world that has lost its humanity at the hands of Quirks. Yoichi refused to give in to his brother's manipulative statements and tried to rebel but was constantly stopped. All For One berated his brother for not having a Quirk, but says that he still loves him as he is his only family, and imprisoned him rather than kill him.

All For One frequently visited Yoichi, who continued to refuse to submit to him and voiced his defiance, to the point where he barely ate food offered by All For One. All For One brags about his successes and says that his dream came true just like in the comic they used to read together. Yoichi replies that his brother had only read through Volume 3 and that there was more to the story, saying that powerful villains always lose to the heroes of justice.

Insulted, All For One reproaches him that real life does not work like in comics, and that he will rewrite reality. Eventually, in a twisted showcase of affection, All For One decided to use his power to forcefully implant a Quirk within him, a Power Stockpiling Quirk that even his frail body could handle, with the intent of turning him to his side.

However, what nobody expected is that the seemingly Quirkless younger brother had, indeed, an innate Quirk, one which could only be transferred to others. His original Transference Quirk and the Power Stockpiling one merged together, and would later become known as One For All. Yoichi would remain imprisoned until he was rescued by two vigilantes who led an assault on All For One's base.

With this new power, Yoichi tried to stop his older brother but he was not strong enough to stop him, so he passed it on to the two vigilantes, who would become the second and third users of One For All. Thus, One For All was passed down to future generations in the hopes that one day One For All it will grow powerful enough to defeat his older brother.

After this, he is presumed to have died in battle against his brother, although he still continues to exist as a vestige within One For All.

U.A. Sports Festival Arc
Yoichi helps Izuku Midoriya for the first time during his confrontation against Hitoshi Shinso in the U.A. Sports Festival's tournament. Hitoshi manages to brainwash Izuku and orders him to go out of bounds, which Izuku obeys.

As Izuku is about to step out of the perimeter of the ring, he has a hallucination and sees eight shadowy figures which causes Izuku to activate One For All and attack himself by snapping his fingers, allowing him to break free of Hitoshi's control and win the fight.

Vs. Hero Killer Arc
After the confrontation with Stain, All Might decides to tell Izuku the origins of One For All and his connection with a villain of great power known as All For One. He tells him how the younger brother of this villain opposed him, and how, out of compassion or looking for a way to subdue him, All For One forcibly gave his brother a Quirk, without knowing that the younger brother already had one. Both Quirks merged and became One For All.

Pro Hero Arc
After an extensive day of One For All training, Izuku returns to Heights Alliance where he falls asleep immediately, experiencing a dream where he sees the previous users of One For All, recognizing them as the vestiges he had seen at the U.A. Sports Festival. Izuku proceeds to view a series of past events regarding Yoichi and All For One that led to the creation of One For All.

However, much to Izuku's shock, Yoichi starts to communicate directly with him. He notes how he wanted to show Izuku more but since his current level can only handle about 20%, it won't permit it, though assures him that he is not alone. When their hands touch in the dream, Izuku wakes up after involuntarily activating One For All, damaging his room.

The next morning, Izuku decides to tell All Might everything, who cannot help but be surprised and concerned to learn that the first user was able to interact directly with Izuku.

Later that day, during a training exercise with his class, Izuku suddenly manifests Blackwhip, the Quirk of the 5th user of One For All, Daigoro Banjo. After re-entering back into the vestige world, he meets Daigoro who reveals to him that the One For All Quirk has evolved to the point that the users can briefly communicate with Izuku, while also telling him that the 6 Quirks of the previous users have also re-emerged, and that Izuku will be able to utilize them as he gets more control of his power.

My Hero Academia: Heroes: Rising
During a devastating battle between a villain with a copy of All For One's Quirk, in a last-ditch effort to defeat him, Izuku transfers One For All to Katsuki Bakugo, even at the cost of rendering himself Quirkless once more. Through the dual power of the One For All's, the two manage to take the villain down, even as the remaining embers of One For All seemingly disappear from Izuku. However, Yoichi and the other users of One For All, wanting Izuku to continue to harness the power, and recognizing his heroism of being willing to sacrifice his power to save everyone, only temporarily transferred the power over to Katsuki, while the full essence of One For All remains within Izuku.

Paranormal Liberation War Arc
Izuku and his classmates participate in an operation to dismantle the Paranormal Liberation Front, although their mission is to evacuate the citizens of Jaku City while a large force of heroes led by Endeavor is tasked with raiding Jaku General Hospital. This is where Tomura Shigaraki is located, who was undergoing a painful and long operation at the hands of the Doctor Kyudai Garaki, ally of All For One and responsible for the creation of the Nomu, to become even more powerful. In addition to making Tomura's body even more powerful and empowering his Decay, he also transplanted All For One's Quirk into his disciple. Now they only needed time to stabilize, but the heroes start the raid before it's completed.

As Izuku and the students evacuate the citizens, Endeavor and his squad arrive at the secret laboratories below the hospital after defeating all the Nomu that Dr. Garaki releases in an attempt to stop them. However, when the heroes think they have already won, at that moment Tomura Shigaraki wakes up. Despite the distance between the two, Izuku gets a bad feeling that something terrible is coming. Soon, the voice of Yoichi, whom he hadn't heard from since before the Joint Training Battle, begins to warn him of Tomura's awakening, and urges him to stop him. When Izuku fixes his gaze on the Jaku Hospital, the building suddenly crumbles to dust as a result of Tomura using his empowered Decay, killing dozens of heroes in the process. The disintegration begins to spread rapidly, reaching in a short time the city itself.

Izuku and his companions manage to escape the destruction in time with the civilians, but later, when listening to a radio communication between Endeavor and Burnin, he discovers that Tomura Shigaraki is going after him, or more specifically, he is going after the One For All. Knowing this, Izuku runs away as far as possible so as not to put his classmates and civilians in danger, being followed by Katsuki.

Tomura soon catches them up, leading to an intense battle in which they, and several heroes, try to stop Tomura. However, thanks to Kyudai Garaki's operation, Tomura is much more powerful than before, and manages to seriously injure Gran Torino, Eraser Head, and Katsuki. Because of all of this, Izuku goes berserk on Tomura, who in turn tries to prevent All For One's consciousness from taking over him. During the struggle, "Tomura" manages to put his hand on Izuku's face and process to try and extract One For All, as Izuku and Tomura find themselves in a mindscape.

Izuku finds himself back in the mist-covered wasteland of his dreams, and in front of him, he sees All For One half fused with Tomura, trying to take control of his body as Tomura tries to reject him. Nana Shimura appears to help him, and both All For One and Tomura are aware of their presence. All For One mocks Nana, while Tomura recognizes his grandmother, who he calmly says he despises her too. Tomura's anger causes One For All to erode, while at the same time trying to destroy his master's manifestation. The eroding stops as All For One realizes his rage is not enough, as Yoichi appears alongside Nana to stop the decay. All For One says it is because his brother is just as obstinate as himself. Yoichi seems to understand what he is using Tomura for, to which All For One blames him for that he could never make him his.

Yoichi proclaims he does not intend to allow him to harm the boy they have chosen as their new heir, as he and Nana start to push the influence of All For One back. All For One tries combining his power with Tomura to fight back and steal his brother's Quirk, while he insults and derides Izuku, claiming that his brother made a mistake for entrusting One For All in the hands of an inexperienced boy. Yoichi, however, rebuts All For One's arguments by praising Izuku's resolve to save people no matter what, and deems him worthy of wielding the Quirk, and they will follow him because of it.

After hearing these words, Izuku's manifestation tries to move towards Tomura one more time. In a grand explosion, Izuku wakes up from the dream world as he and Tomura fall away from one another in the sky. Izuku realizes Tomura failed to steal One For All, as All For One tells Tomura that the reason why was because his body hasn't fully adapted to his new Quirk.

Despite the heroes putting up a fight, All For One manages to take full control of Tomura's body, and uses it to escape the battlefield, as everyone is forced to deal with the ramifications of the costly battles. Through the exhaustion of the battle, Izuku passes out and is taken to Central Hospital, alongside most of the other injured heroes, to heal. However, unlike most of the others who had awaken after several days, he still remained unconscious.

This is because his mind was back in the vestige realm, where he found himself faced with all the previous users of One For All. Yoichi reveals to Izuku that originally they were only able to interact with him during specific instances, but since the power has grown exponentially over the past couple months, the vestiges are now able to more freely communicate with each other.

The users also reveal to Izuku the revelation that they had discovered through conversing with All Might's vestige and examining his real life counterparts investigations: the 4th user, Hikage Shinomori, who held One For All for 18 years and died at the age of 40, died due to possessing multiples Quirks which ate away at his life. Yoichi points out that due to all the other users dying young and in battle, there wasn't much to compare Hikage's situation, especially considering All Might held onto the Quirk for much longer with no issues, until they all realized the one major difference; All Might was Quirkless.

Yoichi goes on to explain how the spirits of the predecessors within One For All are the user's original Quirk Factors, with All Might being the sole exception as One For All because his true Quirk. He continues that a vessel with an existing Quirk could not receive One For All without it warping and spilling over, and attempting to force it results in a shortened lifespan, like what happened with Hikage.

They conclude that due to these circumstances, the true value of the power can only be wielded by those who lacked power to begin with, with Izuku following up that especially now that the previous user's Quirks have started manifesting, One For All can not be given to someone with a Quirk. Yoichi says that due to Quirkless individuals slowly vanishing each year, it was fate that Izuku managed to be the one who was bestowed the power, and that if they had a way to contact him sooner, they could've prevented more people from dying. Daigoro adds that because of this, Izuku may end up being the last successor of One For All.

After Izuku's contemplation of this revelation, Nana Shimura turns to him, asking if he can kill Tomura Shigaraki, clarifying it isn't a request, but a question of his resolve. She reminds him of their last confrontation with him, and that since all of the inheritor's are connected to his emotions, they could sense that he saw him as someone who needed saving.

Yoichi tells Izuku that he believes his brother purposefully raised Tomura to be completely filled with hatred as part of another plan to claim One For All. The users reveal that All For One had attempted to steal One For All from them twice, but failed both times, and that one must have an incredibly strong will and overbearing emotions in order to override the basic principle of One For All, which means All For One is using Tomura's hatred to complete the job. The inheritors tell Izuku that One For All was itself a powerful will meant to resist All For One, and that they have been cultivating it for the specific purpose of destroying him.

Despite the warnings of what could happen if Tomura manages to reach his complete form, Izuku tells the users that when Tomura's conscience manifested in One For All, he could see and feel deep down within him, an image of his younger self, as Tenko, crying. He tells the users how he never got the chance to understand what drove the Villains to end up that way, and wonders if the resolutions would've ended up differently if he did.

He declares that despite everyone Tomura has hurt, he stands his ground believing One For All is a Quirk meant to save lives, not kill, and that All Might and all the users who have helped grow and cultivate the power, have in the process saved and brought hope to countless people, and that One For All's purpose has changed overtime to become more than just defeating All For One. He tells them he understands he may have to kill Tomura, and he doesn't know how he will deal with everything exactly yet, but he still wants to try and save "that little boy". Yoichi happily accepts Izuku's answer, telling him that he and the others will be there for him, and that he's grateful that he and All Might inherited their power, as he comforts the crying vestiges of All Might and Nana.

Yoichi turns towards the second and third users of One For All, who had their faces up against the back wall the entire time, telling them to play nice and cooperate with Izuku, as he'll need their help to unleash the Quirk's full power. After they react in silence, the other users proceed to converse with Izuku, while Yoichi speaks to the second and third.

The third user talks about their time living in the era when All For One's control was at its peak, and how their leader, the second user, did everything in his power to strike back against his rule. The second user talks to him about how many lives he took in order to bring down All For One, and that he can’t get behind devoting their efforts to a boy who wants to “save” their mortal enemy, calling it a ridiculous delusion.

Yoichi then asks why the two users saved him despite their initial plan to kill him due to his ties to their “enemy” when they found him. He tells the second user that if it wasn't for him rescuing him from his room, One For All would never of been born in the first place, and reassures him that, with all of them behind him, Izuku will be the one to finally be able to bring an end to All For One's terror.