Hamaguchi, a middle school classmate and close friend of Nishikata, Takao, and Kimura at Tonoshō Municipal Tonoshō Junior High School, sports fair skin, light brown tousled hair, and grayish eyes, standing out among peers with his taller, slightly athletic frame. His wardrobe alternates between a white button-down shirt paired with navy blue pants and a black button-down shirt featuring yellow buttons in colder months.

Beneath his kind demeanor lies a nervous streak, most evident around his crush, Houjou, the school’s popular figure. Though mutually drawn to each other, both initially bury their feelings. Hamaguchi strains to project maturity, choking down bitter coffee and jogging relentlessly to join a sports club at her casual suggestion—clumsy endeavors that underscore his endearing awkwardness. Their bond inches forward when he musters the courage to invite her to a summer festival, where shared laughter under fireworks hints at deepening affection.

A later live-action film, set a decade post-graduation, reveals their bittersweet arc: the pair dated but eventually parted, with Houjou rebuffing his attempts to rekindle their relationship—a stark contrast to their timid middle school connection.

Hamaguchi thrives in his friendships, bonding with male peers over fishing trips and mutual romantic grievances. He once orchestrated a scheme pressuring Nishikata and Takao to confess their crushes, determined not to navigate love alone. Their dynamic balances camaraderie with playful peer pressure, absent of serious conflict.

Consistently portrayed across adaptations, Hamaguchi embodies adolescent earnestness, his stumbles and small triumphs reflecting varied timeline outcomes in spin-offs while staying anchored in the awkward, heartfelt pursuit of connection.