Hiroki is a courteous and cool elementary school student in the same grade as Kakeru. Born into wealth, his affluent background subtly influences his behavior and resources, such as carrying a non-functional golden net modeled after the standard Monkey Net. His friendship with Kakeru thrives alongside a fierce rivalry in monkey-catching activities, where Hiroki constantly competes to prove his skills, highlighting his determined nature.

He harbors overt and persistent romantic feelings for Natsumi, the Professor's granddaughter, carrying her picture and dreaming about her regularly. This infatuation drives many of his actions, including heroic moments like shielding Natsumi from Pink Monkey's attack in Episode 36 of Season 2. Hiroki actively participates in missions to capture Specter's monkeys alongside Kakeru, Natsumi, and the Professor, demonstrating loyalty and protective instincts—particularly toward Natsumi—even at personal risk.

Throughout the anime adaptation, Hiroki maintains core traits of competitiveness, affection toward Natsumi, and underlying responsibility. His wealthy upbringing occasionally surfaces through his demeanor and possessions but never overshadows his role within the group dynamic.