Kotaro Yanagisawa

For most of his appearances he stays completely hidden inside the Shiro disguise, a hooded set of pale robes and a full mask woven from anti-sensei fibers, with the emblem of a hydrogen Bohr model printed across his back. A device distorts his voice so that Korosensei cannot recognize him.

Strip away the costume and Yanagisawa is revealed as a fair-skinned man whose short, wavy hair is a dark green above eyes of pale teal. His left eye is an artificial replacement, lost during the confrontation that scarred his past. Once he floods his own body with altered tentacle cells, he tears off the robes to expose a black tank top, his skin crawling with dark veins while the muscle of his arms swells into rope-like shapes that can hide a syringe. The manga blackens the whites of his remaining eye; the anime only drains its color.

Yanagisawa first comes across as playful and relaxed, a manner he keeps up even while helping Itona Horibe assault Korosensei, but the friendliness is a shell over a scheming, manipulative, and pitiless core. Nearly every other person registers to him as something expendable. In his years as a lead researcher he was coldly indifferent, content to experiment on human test subjects and to vent his frustration on the staff beneath him, his fiancée Aguri Yukimura included.

Injecting himself with tentacle cells sharpens his cruelty to a razor. His obsession with destroying Korosensei grows so total that his own survival stops mattering to him, and he can openly laugh at Kaede Kayano being mortally wounded, remarking how amusing it is that she falls just as her sister did. That fixation exposes a hunger for a flawless result, which is why both Akari Yukimura and Itona became vessels for his tentacles.

Yanagisawa was the lead mind on a project attempting to generate antimatter inside a living organism, sidestepping the enormous energy such matter normally demands. Born to wealth and celebrated as a biology prodigy at the head of a global research body, he nonetheless confessed that he had no real drive of his own. As a university student he crossed paths with Aguri Yukimura, an aspiring teacher whose family firm, Yukimura Pharmaceuticals, was failing. He bought the company and arranged to marry her, calculating that her fierce dedication might compensate for the fire he lacked, though he claimed publicly to have taken her in out of charity and treated her with contempt.

His experiment needed a living host, so he secured a condemned killer known as the Reaper and set the disposable Aguri to watch over the man his colleagues feared. To gauge how the antimatter cells would behave once a host aged and died, he rocketed an identically injected mouse to the moon. The mouse perished, its cells ran wild, and the resulting reaction tore away the bulk of the lunar surface, leaving the moon its lasting crescent shape. Terrified the same fate awaited his human subject, he ordered the Reaper destroyed. When Aguri, overheard begging the prisoner to survive, betrayed his expectations, his wounded pride boiled over and he struck her down. The laboratory blast that followed cost him his left eye but not his life.

Learning that his former test subject was now teaching Class 3-E as Korosensei, Yanagisawa raised an abandoned boy, Itona Horibe, and grafted tentacles onto him as a weapon. Repeated failures with Itona led him to cast the child aside, and he later shifted to framing Korosensei and to steering a superweapon program, taking custody of the beaten Reaper. In the final stretch he unmasks, pairs with a counterfeit Reaper capable of Mach 40, and injects himself with tentacle cells to join the assault. He orders the false Reaper to run Kayano through and revels in Korosensei's rage, only for the teacher's radiant final surge to annihilate his monster and hurl him into the anti-tentacle barrier. He survives, but crippled for life, stripped of his inheritance, his research abandoned as useless, left entirely alone.