Kotetsu, a cat distinguished by a prominent moon-shaped mark on his forehead, belongs to Chie Takemoto. Originally a stray, he approached the Takemoto family's izakaya appearing hungry. Chie fed him, leading Kotetsu to move in permanently. He exhibits a capacity for unexpected violence, notably during an encounter with Shachou's cat, Antonio. In a confrontation instigated by their owners, Kotetsu defeats Antonio and partially castrates him. This act profoundly affects Shachou, contributing to his decision to leave the yakuza and open an okonomiyaki restaurant, while establishing Kotetsu's reputation for aggression – a trait later noted by Tetsu Takemoto when complaining about these "violent tendencies" at Shachou's restaurant.

A significant shift occurs when Antonio Jr. arrives seeking revenge. Despite expectations of another fight, Kotetsu defuses the situation by showing respect for the deceased Antonio and refusing combat. This peaceful resolution leads Antonio Jr. to forgive him. Kotetsu remains a consistent presence in the Takemoto household across adaptations, living alongside Chie, her father Tetsu, and eventually her mother Yoshie after their reconciliation. Referenced as part of the "comical and cheerful everyday life" in the sequel television series, he maintains his role as a companion animal within the family.