Sonomura is a student in Yamaguchi Kumiko's class. He is first introduced in chapter 1 with his fellow classmate though not by name. His name is finally revealed in chapter 3 of the manga (episode 2 of the anime) when he and a classmate were fighting. During the fight, Sonomura pulled out a switch blade from his pocket, which he would have used against his classmate. Fortunately for him and his opponent, Yankumi intervened and twisted the blade out of his hand.