Hamaguchi is a tall, fair-skinned junior high student with a slightly athletic frame, tousled light brown hair, and gray eyes. He alternates between a standard uniform of white button-down shirt and navy pants or seasonal wear like a black shirt with yellow buttons paired with dark trousers. His social circle centers on classmates Nishikata, Takao, and Kimura, with whom he shares fishing excursions, schoolyard conversations, and commiserations over romantic challenges.

A childhood friend of popular Houjou, he nurses quiet affection for her, mistaking bitter canned coffee consumption and impromptu jogging sessions—spurred by her offhand sports club remark—as markers of maturity. Their interactions oscillate between clumsy confession attempts and fleeting intimacy: shared walks, festival visits, and unspoken tension that edges toward connection. While the manga concludes with them attending a summer festival together, implying mutual interest, the anime preserves their unresolved dynamic. The live-action adaptation departs sharply, portraying a turbulent post-graduation romance that dissolves before their ten-year reunion, where Hamaguchi tentatively seeks reconciliation.

Socially earnest yet privately anxious, he oscillates between rallying friends to pursue their crushes and retreating into self-doubt. His loyalty surfaces in supportive peer interactions, though Houjou’s presence often reduces him to nervous gestures masking vulnerability. The character remains absent from the spin-off series centered on Nishikata’s adult family life, his narrative confined to earlier youth-focused iterations of the story.