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Despite Satoru succeeding, by the time Gaku is reveals himself to be the killer, Satoru ended up taking the place of the three original victims on the day the killing. However, his plans would have succeeded if not for Satoru, who used " Revival" to travel back in time to stop him, manipulating events in order to prevent their deaths. There, he attempted to kidnap and kill Kayo Hinazuki, Hiromi Sugita, and Aya Nakanishi. He eventually became the home room teacher of grade 5, class 4 at Satoru Fujinuma's elementary school. The police determined that she killed herself over stress. He ended up killing her by pushing her out the window.
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As children began to go missing, she became suspicious and started asking him questions about what he was during on the days of murders. But, he wasn't interested in her, but how she could help him in his path. She majored in clinical psychology and child development. Three years later, in his second year of teaching at Sapporo, he found himself engaged to a psychological counselor who was two years older. In the town, he tried to abduct another girl, but failed when her friend showed up, which lead Gaku to make a more cautious approach in future. In his third year of college, he went back to his hometown as part of a two-week teaching practice. He went onto college to become an elementary school teacher. Gaku moved into his grandfather's house along with his mother. Soon afterwards, his parents divorced and sold their house. His mother discovered his body first and the police ruled that he killed himself out of guilt. Gaku ended up doing Fratricide by murdering his brother and making it seem like a suicide. Gaku was allowed to walk home with just his brother. Children now had to walk home in a group or with a guardian to keep safe and there was a curfew in effect. The news of the girl's disappearance became a popular topic in school. Afterwards, they both hid the girls body temporarily in the storehouse. As he heard his mother talking, he tried to silence the girl by covering her mouth, accidentally suffocating her. He waited for her to leave before entering the storehouse. When he got back, he found his mother outside of the storehouse talking to someone. Thinking no one would come, he left his hamster there and went to play. On his first day of morning middle school, his brother ordered him to keep watch outside of the storehouse. After his brother assaulted the girl it would be Gaku's job to comfort them so they would keep quiet about it. He would lure girls from his school into his family's storehouse. During his fifth year, his brother convinced him into becoming his accomplice.
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He went to Mikoto Elementary School along with his brother. They placed all their expectations onto Gaku, neglecting his older brother and causing Gaku to be abused and manipulated by him. Gaku was born to mild parents with a brother that was two years older than him.
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Yashiro carefully thought out his plan that he killed Hiromi Sugita, a feminine-looking boy, to make it appear as if the killer's only interest was in females by mistaking the boy for one. Gaku plans out all of his murders and ensures no evidence leads back to him while distancing himself from the incident in one town before moving to a new one. His desire to kill seemed to stem from his childhood along with an experience where he drowned a group of hamsters and saw one alive while standing on one of the corpses of his fallen brethren. He also killed those who he saw had a spider string above their heads. He was very methodical, ensuring two conditions were met: first, that he had someone he could frame and second, there would be a period where the person was alone. However, beneath this facade laid a deplorable, sadistic amoral psychopath who enjoyed killing. He even helps out Kenya Kobayashi, Osamu, Hiromi Sugita and Kazu with their birthday scheme by making Kayo and Satoru stay late at school so that the group could prepare surprise. In addition, he tries to defend his students when they are wrongly accused, such as when Misato Yanagihara accused Kayo Hinazuki of stealing her lunch money. Initially, Gaku is presented as a kind, charismatic and understanding teacher who is popular with his students as he is always willing to help out. He often wears a bowler hat, brown square-rimmed glasses, and black gloves to obscure his identity. In his later years, his hair is slicked back and has some white streaks. Gaku is a fair medium-skinned man with short black hair that has bangs hanging over his eyebrows, brown eyes and he is always seen wearing a suit.
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