Yoichi Shigaraki/Synopsis

History
Yoichi was born in a time when the Quirks began to manifest for the first time in human society. Apparently, he was born 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 his weak body, he could hardly do anything but watch helpless as his older brother commit 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 him 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 refuses to give in to his brother's manipulative statements and tried to rebel but was 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.

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. All For One brags about his successes and says that dream come true just like in that comic they used to read together. The younger brother replies that the elder has only read through volume 3 and there was more to the story. He says that powerful villains always lose to the heroes of justice.

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 used his power to forcefully implant a Quirk within him, a stockpile power 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, a innate Quirk, one which could only be transferred the user's powers to others. His original Quirk and the Quirk that was implanted to him by All For One merged together, and would later become known as One For All. Yoichi remained 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 became 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. It is suggested that All For One would fight these successors.

After this, he died in unknown circumstances, although he still continues to exist as a vestige in the 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 faced him. Even thus, 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
In Heights Alliance, he appear again. Izuku is exhausted from training and falls asleep immediately. He experiences a dream where he sees the previous users of One For All - He recognizes that this is similar to the vision he had during the U.A. Sports Festival. Izuku views the past events that led to the creation of One For All.

Much to Izuku's shock, the first user appears and communicates directly with him. Yoichi notes he wanted to show Izuku more but Izuku's current level won't permit it, though he assures Izuku 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.

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 Deku 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 the first user of One For All, 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, for some reason, 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 Bakugo.

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 all of this, Izuku goes berserk on Tomura, who in turn tries to prevent All For One's conscious takes over him. During the struggle, "Tomura" manages to put his hand on Izuku's face and process to extract the One For All. But all does not go as planned, as the move brings Izuku and Tomura into 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, he calmly tells her he despises her too. Tomura's anger causes One For All to erode, at the same time that he tries to destroy his master's manifestation. The eroding stops as All For One realizes his rage is not enough, as his younger brother, the first user of One For All, appears as well and stop the decay. All For One says it 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 starts 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 insults and derides Izuku, claiming that his brother made a mistake for entrusting One For All in the hands of an inexperienced boy. The first user of One For All 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, Deku wakes up from the dream as he and Tomura fall away from one another in the sky. Deku 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.