Yōhei Yamada, the young human brother in the Yamada family, discovers the lost kitten Chi during a park visit. He plays a pivotal role in naming her "Chi," inspired by her initial bathroom mishaps and derived from the Japanese word for urination. His family resides in an apartment complex initially prohibiting pets but decides to keep Chi regardless.

Yōhei exhibits a cheerful, kind, and obedient personality. He shares a close bond with his parents: his father, Kento, actively supports him and engages in solving his problems, while his mother, Miwa, prioritizes his well-being, fostering mutual attachment and respect. Yōhei displays a particular fondness for toy trains. His relationship with Chi is deeply affectionate; he becomes emotionally distraught and tearful at the prospect of moving away and potentially leaving her behind.

Across the series, Yōhei matures noticeably. Initially appearing very young, estimated around six years old, he is characterized by bright red-orange hair and a highly childlike demeanor. By the time of the 2016 anime continuity, his age is suggested to be around ten years old. His hair color shifts to golden-brown, aligning with his increased maturity and involvement in family activities, such as assisting his mother in preventing Chi from walking on new carpets with dirty paws.

Yōhei interacts with characters beyond his immediate family. His initial encounter with the neighborhood cat Blackie involves fear, comparing him to a bear. He also forms a connection with Cocchi, another cat who becomes Chi's friend. While his direct adventures are less documented, his presence remains integral to Chi's experiences within the Yamada household and their pet-friendly apartment.