Yōhei Yamada becomes Chi's primary human companion after finding her lost in a park. Contextual clues indicate he was roughly 3-4 years old during early episodes, shown potty training alongside Chi. His visual design evolves across adaptations, depicting noticeable maturation in later series like *Chi's Sweet Adventure*, where he appears approximately 10 years old.

Yōhei consistently demonstrates cheerfulness, kindness, and obedience, with a recurring fondness for toy trains. His discovery of Chi stems from empathy; he felt distraught witnessing her distress and acted on sympathy to bring her home despite his family’s apartment initially banning pets.

Within the Yamada family, Yōhei shares a close bond with both parents. He relies on his father, Kento, for task assistance and problem-solving, while showing deep attachment to his mother, Miwa. He typically follows her guidance, and she prioritizes his interests. His relationship with Chi forms an emotional anchor—he actively participates in her care and expresses intense distress during potential separations. He once protested a family move by exclaiming, "No way! I ain't going!... That means I can't see Chi... I always really wanted to be with Chi!" This reaction directly influenced their relocation to a pet-friendly home.

His development includes deeper engagement with Chi’s world. A series of "Mame-Chi-Shiki" promotional tweets framed as his reactions to new *Chi’s Sweet Adventure* episodes featured him sharing cat facts, occasionally researching unfamiliar topics to proactively understand feline behavior.

Physically, his hair color shifts from red-orange to golden-brown across adaptations. Though primarily a supportive household figure, his interactions with Chi consistently reflect nurturing instincts and emotional dependence on their bond.