Michio Yūki is the central figure of the manga MW. He presents himself as a handsome, intelligent, and impeccably dressed loan officer at a major bank, the scion of a distinguished Kabuki family. Beneath this flawless exterior, however, he is a psychopathic serial killer, kidnapper, and rapist who feels no remorse or empathy. His entire moral compass was obliterated during a childhood accident when he was exposed to a leak of the secret nerve agent MW on Okino Mafune Island, an incident that killed everyone else present and was subsequently covered up by authorities. The gas left him with lingering physical aftereffects that progressively damage his brain and heart, and he becomes acutely aware that his life will be cut short.

Michio’s primary motivation is a cold, calculated desire for revenge against those who orchestrated the cover-up of the MW disaster. As his condition worsens, this goal expands into a far more nihilistic ambition: to obtain the remaining supply of MW and release it upon the entire world, wiping out humanity as his final act. He pursues this objective with methodical precision, using his charm, intelligence, and cunning to manipulate everyone around him. He is a master of disguise, using his Kabuki training to convincingly impersonate women, often taking on the identity of his latest female victim. He also leverages his sexuality without hesitation, engaging in relationships, both genuine and transactional, to gain access to people and information.

His most significant relationship is with Father Garai, a Catholic priest who was also a survivor of the island incident and with whom Michio shares a deep, lifelong bond. This relationship is complex and intimate; Garai is both Michio’s confessor and his reluctant enabler, torn between his love for Michio and his horror at the crimes he commits. Throughout the story, Michio remains a static and irredeemable figure, accelerating his descent into violence as his body fails him. His abilities extend beyond disguise to include a sharp tactical mind, a gift for social climbing, and an utter lack of conscience, which makes him capable of any act without hesitation or guilt.