Tomoyo Yamakawa serves as a supporting character within the narrative and belongs to the Yamakawa family, which also includes Takeo Yamakawa. Her specific role becomes significant when protagonist Musuke "Shorty" Sato finds himself in trouble after lending entrusted money to the school bullies Tower and Dump. Facing this predicament, Musuke seeks help from Tomoyo, who is identified in this context as the pitcher's sister.