Michio Yuki was once a sweet and empathetic kid who had a good future. He is the younger brother of Tamanojo Kawamoto, who is also just like him. He was a 4th grader at Hanasaki elementary. However, it all changed for the worst, when Michio is placed on Mafune Island and met the teen Garai. On the day on the island, he was kidnapped by Garai himself in a cave and the two have formed a homosexual relationship there. By the time they went out of the cave, they found themselves being survivors of a leak from a chemical warfare known as MW. Howver, Michio got affected by the gas, turning him insane. Since the incident, Michio works as a bank employee by day and becomes a mass murderer at night.
Michio plans get revenge on everyone connected to the MW incident, but in reality, his plan is to get the titular gas so that he spread it around the world when he dies.
- All of Michio's characteristics are similar to Johan Liebert, the main antagonist of the manga Monster. They were once innocent kids, but turned insane when affected by secret military experiments, can manipulate people, have ambitions to end the world, try to commit suicide by cop to prove that their nihilism succeeds over their killer's idealism.