Gaku Yashiro

Gaku Yashiro is Satoru's homeroom teacher in primary five, and the real kidnapper behind the scenes. After Satoru's attempt at stopping his actions, he decided to drown Satoru in a frozen lake, which resulted in Satoru's 15 years coma. In the 2006 timeline, he is going by the name Nishizono Manabu, which is rather an obvious identity for Japanese speakers (Manabu and Gaku share the same Kanji).

History
Gaku was born to mild parents with a brother that was two years older than him. They placed all their expectations onto Gaku, neglecting his older brother and causing Gaku to be bullied by him. He went to Mikoto Elementary school along with his brother. During his fifth year, his brother convinced him into becoming his accomplice. He would lure girls from his school into his family's storehouse. After his brother assaulted the girl it would be Gaku's job to comfort them so they would keep quiet about it.

On his first day of morning middle school, his brother ordered him to keep watch outside of the storehouse. Thinking no one would come, he left his hamster there and went to play. When he got back, he found his mother outside of the storehouse talking to someone. He waited for her to leave before entering the storehouse. Inside, he found his brother panicking. As he heard his mother talking, he tried to silence the girl by covering her mouth, accidentally suffocating her. Gaku's brother tried CPR to no avail. Afterwards, they both hid the girls body temporarily in the storehouse.

The news of the girl's disappearance became a popular topic in school. Children now had to walk home in a group or with a guardian to keep safe and there was a curfew in effect. Gaku was allowed to walk home with just his brother. Gaku ended up murdering his brother and making it seem like a suicide. His mother discovered his body first and the police ruled that he killed himself out of guilt.

Soon afterwards, his parents divorced and sold their house. Gaku moved into his grandfather's house along with his mother. He went onto college to become an elementary school teacher. In his third year of college, he went back to his hometown as part of a two-week teaching practice. 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.

Three years later, in his second year of teaching at Sapporo, he found himself engaged to a psychological counselor who was two years older. She majored in clinical psychology and child development. She was a guest lecturer at his college. She taught him about child development. But, he wasn't interested in her, but how she could help him in his path. As young girls began to go missing, she became suspicious and started asking him questions about what he was during on the days of murders. He ended up killing her by pushing her out the window. The police determined that she killed herself over stress.

He eventually became the home room teacher of grade 5, class 4 at Satoru's elementary school. There, he tried to kidnap 3 other children but his plans were foiled by Satoru.

In 2003, he was a politician, going by a new alias. In the manga, he entices Satoru to a camp he made, using Satoru's friend Kumi as bait. Satoru reveals to him, during their confrontation, that he has seen the future and has changed it, which Gaku believes. He attempts to kill himself along with Satoru but Satoru saves him, and he is then arrested. He admits to his murders and is sent to Chiba Prison. In the anime, after meeting a disabled Satoru, he triggers Satoru's memories, remembering Gaku as the kidnapper. His plan to kidnap a child at the hospital once again failed, thanks to the efforts of Satoru's friends. After Satoru seemingly commits suicide by going over a rooftop, Gaku proceeds to do the same, but finds Satoru had been safely caught by Satoru's allies. Gaku is then finally apprehended.