|Birthday||June 16, 1966|
Satoshi has spent much of his life protecting his younger sister Satoko. Their mother has married and divorced several men, leaving the pair with a troubled family life. Near the end of every marriage, their mother starts to lash out at Satoshi and Satoko, causing friction within the family. After the siblings' parents rebel against the Sonozaki family, practically every villager in Hinamizawa begins to treat the siblings with disdain. After the pair's parents die on vacation in June 1980, Satoko and Satoshi end up in the care of their aunt and uncle. Both their aunt and uncle are abusive towards the siblings. Satoko frequently fights with their aunt and Satoshi protects Satoko from their aunt's anger. In order to cope with the stress of his home life, Satoshi joined the local baseball team, the Hinamizawa Fighters, managed by Kyousuke Irie.
In 1982, Satoshi developed symptoms of Hinamizawa Syndrome. He becomes consumed with the fear that he will be the victim of the fourth Watanagashi Festival. As Satoko's birthday approaches, Satoshi takes a part-time job to earn money to buy her a teddy bear as a present. To make time for his job, Satoshi quits the Hinamizawa Fighters. When he speaks to Irie about leaving the team, he asks if he can keep his baseball bat under the pretense of practicing his swing. That June, Satoshi beat his aunt to death with his bat. A few days afterward, Satoshi asks Irie to meet him outside a toy shop in Okinomiya to help him move the teddy bear he bought. Irie picks up Satoshi and loads the teddy bear in his car. During the car ride home, Irie notices Satoshi showing symptoms of the terminal stage of Hinamizawa Syndrome. Irie drives Satoshi to the Irie Clinic for testing. After being anesthetized, Satoshi disappears.
Satoshi appears as a young blonde male with red-violet eyes. He usually wears his school uniform with a white shirt over a white undershirt and blue slacks with a brown belt.