Kotarō Yanagisawa, initially known by the alias Shiro, is a central antagonist in the story. He is a brilliant but ethically bankrupt scientist who comes from a wealthy family that runs a bio-company. From a young age, he showed remarkable scientific aptitude but lacked empathy or moral restraint, viewing other people as tools to be used for his own ends. His personality is marked by arrogance, a coldly manipulative demeanor, and a deep-seated belief that his genius justifies any action. He presents a calm and easygoing exterior when it serves him, but this masks a ruthless nature; he is willing to sacrifice anyone, including his own fiancée and child allies, without hesitation. His primary motivation is a wounded pride and a burning desire for revenge against Koro-sensei, the test subject who escaped his control and destroyed his life's work, leaving him disfigured and humiliated. He is driven not by a scientific ideal but by a personal vendetta to reclaim his lost reputation and prove his superiority. In the story, Yanagisawa is the direct cause of the central conflict: his experimental anti-matter research created the tentacles, transformed the assassin known as the God of Death into Koro-sensei, and indirectly caused the destruction of seventy percent of the moon. He acts as the hidden puppeteer, first manipulating the boy Itona Horibe by implanting him with tentacles and using him as an assassin, and later secretly observing and advising his sister-in-law Kaede Kayano as she plots her own revenge. When both of these indirect methods fail, he takes a more direct role, eventually implanting tentacles into his own body to confront Koro-sensei personally in a final, desperate battle. His key relationships are defined by exploitation. His arranged engagement to Aguri Yukimura was purely transactional, and he abused her verbally and physically, seeing her as a disposable asset. Her eventual death fuels part of his hatred, though he blames Koro-sensei rather than himself. He uses Itona Horibe as a weapon, discarding the boy coldly after repeated failures. He has a complicated history with Kaede Kayano, Aguri's younger sister, acting friendly toward her when she was a child but later manipulating her as another tool for assassination. His relationship with Koro-sensei is one of obsessive hatred; he cannot forgive the former test subject for escaping, injuring him, and causing the collapse of his project. Over the course of the story, Yanagisawa's character develops from a shadowy schemer working through proxies into a desperate fanatic who sacrifices his own humanity. He becomes increasingly unhinged as his plans fail, culminating in a final act where he injects himself with tentacles, transforming into a monster. This transformation ultimately leads to his defeat, as Koro-sensei's fury results in Yanagisawa being critically injured, left permanently bound to a life-support system, stripped of his power and ambition. In terms of abilities, Yanagisawa is a genius scientist and the creator of the tentacle technology. He develops a range of anti-sensei weapons and devices, including specialized fibers for his white suit and mask, prosthetic eyes with offensive capabilities, and voice-changing technology. He is also knowledgeable about Koro-sensei's weaknesses and biology, having been the lead researcher on the original experiment. In his final confrontation, he demonstrates physical combat capability enhanced by the tentacles he implants into himself, though this is a reckless and imperfect imitation of the power he sought to control.