Gaku is Satoru's homeroom teacher during primary school.

Spoiler

In the original timeline, Yashiro successfully murdered two children at Mikoto Elementary School—Kayo Hinazuki and Hiromi Sugita—as well as another student from Izumi Primary named Aya Nakanishi. After being a school teacher, he inherited his father-in-law's constituency and became a council member in his place. During his time as a council member, he is seen by Sachiko Fujinuma during his attempt to abduct another child. After Sachiko remembers his identity, she is immediately murdered and her son, Satoru, is framed for the murder.

Spoiler

In the third Revival, while Satoru is unaware of Yashiro's true identity as the murderer, he completely renders Yashiro's plans useless. Yashiro attempts to murder Satoru by drowning him in a car in which he is locked in tight by a seatbelt. Satoru somehow survives, and Yashiro is left entertained about Satoru's persistence. Satoru goes into a coma for 15 years, and Yashiro watches over him until he wakes up again. Upon Satoru's awakening, Yashiro again tries to frame Satoru for murder, only to be caught by a plan formulated by Satoru.