Hiroshi Kaizuka is the main protagonist of the story, an ordinary Japanese middle school boy who becomes the central figure after a bizarre accident. He first encounters the talking frog Pyonkichi when he trips and falls on top of it during a scuffle with his rival. This unusual event leads to the frog being flattened but surviving as a living imprint on the front of Hiroshi's shirt, creating an inseparable and often contentious partnership.

In terms of appearance, Hiroshi is a young boy with black hair that reaches his ears and matching black eyes. His most recognizable trademark is the pair of sunglasses he habitually wears perched on top of his head. Interestingly, despite his school enforcing a uniform dress code, Hiroshi is permitted to wear his regular shirt instead, a small but notable detail that hints at his unique circumstances or perhaps his stubborn nature.

Personality is the defining aspect of Hiroshi Kaizuka, built around a series of sharp contrasts. He is described as quite gutsy, a boy who will not back down from a fight or an argument, showing considerable bravery and a hot-blooded spirit when facing external threats. This is particularly evident in his interactions with his rival, Goriraimo, against whom he shows no fear and is always ready for a confrontation. However, this courage has a very clear and comedic limit, as he is deathly afraid of his mother, referred to as Kaa-chan. This fear of his mother is a dominant trait, instantly deflating his bravado and causing him to lose his nerve completely.

His role in the story is to act as the human anchor for the series, navigating the everyday challenges of school life, friendships, and rivalries while also managing the chaotic presence of Pyonkichi on his chest. The talking frog serves not only as a bickering companion but also as a source of advice and commentary on his actions, forcing Hiroshi to constantly defend his choices and argue his point of view. Their relationship is the core of the narrative, functioning as a constant back-and-forth of insults and disagreements that masks a deep, if unspoken, bond.

Key relationships define his world. The rivalry with Goriraimo is a significant part of his daily life, providing the physical and competitive outlet for his gutsy nature. In contrast, the relationship with Pyonkichi is more complex, combining the annoyance of a nagging conscience with the reliance of a partner. He also shares a strong, if fear-driven, relationship with his mother, whose authority is the only force capable of taming his rebellious spirit. Another notable trait is his casual defiance of school rules, as he is, for unspecified reasons, the only student allowed to avoid wearing the proper uniform shirt.

Hiroshi does not undergo a dramatic, world-changing transformation. Instead, his development is subtle and situational, rooted in the ongoing friction between his brave public persona and his private fears. His notable abilities are not superhuman but practical. He possesses considerable guts and physical bravado when facing peers like Goriraimo. His primary skill, ironically, might be his capacity for argument, as he constantly debates with the assertive Pyonkichi, refusing to be dominated by the frog's opinions. His trademark sunglasses and his uniquely exempted shirt serve as visual markers of his individualistic and somewhat rebellious personality, setting him apart from the conforming students around him.