Hiroshi Mochizuki is a young boy who serves as a central figure in the narrative. He lives with his grandfather, Seikichi Mochizuki, a local inventor, following the mysterious disappearance of his father, Doctor Mochizuki, a brilliant but misguided geneticist. This absence leaves Hiroshi with a deep, unspoken longing to be reunited with his father, a desire that underscores his emotional state throughout the story.

In terms of personality, Hiroshi is initially presented as an ordinary child who finds himself thrust into extraordinary and terrifying circumstances. Being targeted by monstrous creatures created by his father's失控 experiment, the Neo Organism, would frighten anyone, and Hiroshi's initial reactions are defined by fear and confusion. This is most evident in his first encounters with Masaru Aso, the man who can transform into the grasshopper-themed warrior Kamen Rider ZO. Hiroshi is scared of ZO, not understanding this being's intentions. However, as ZO consistently protects him and proves himself to be an ally, Hiroshi's fear gives way to trust and affection. He begins to refer to ZO as Onii-chan, a Japanese term meaning older brother, signifying the deep bond of protection and familial care that forms between them.

Hiroshi's primary motivation is simple but powerful: personal connection. He is driven by a child's yearning for his missing father and a need for safety from the forces hunting him. The most significant memento from his father is a pocket watch that plays a distinct melody. This small object is not just a keepsake; it becomes a crucial plot device that acts as a link to his father and ultimately proves vital in the final battle against the Neo Organism. The music from the watch distracts the creature at a critical moment, allowing ZO to break free from being absorbed and achieve victory.

His role in the story is that of a protected innocent, the human heart of the conflict. The monstrous Doras, the physical embodiment of the Neo Organism, relentlessly pursues Hiroshi to use him as leverage to force his father to complete its evolution into a "perfect lifeform". Hiroshi is the prize in the battle between Kamen Rider ZO, who is driven by an unconscious urge implanted by his father to protect the boy, and the unfeeling, monstrous creations of his father's ambition. His presence humanizes the conflict, transforming it from a simple battle of good versus evil into a personal struggle to protect a child's future.

Key relationships define his world. The most important is with Masaru Aso, the lab assistant turned cyborg hero. Where Masaru's own creator (Hiroshi's father) sees him as an experiment, Hiroshi sees him as a brother, creating a found family dynamic that is the emotional core of the film. His relationship with his grandfather, Seikichi, is one of domestic care; Seikichi is his guardian and provides a sense of normalcy and humor amidst the chaos. The most fraught relationship is with his absent father, Doctor Mochizuki, whose scientific hubris set the entire tragedy in motion, but who ultimately repents and sacrifices himself to save his son. Hiroshi's character development is a journey from a vulnerable and frightened child to a resilient figure of hope. He learns to trust a stranger (Masaru), confronts the danger posed by his own father's legacy, and survives the ordeal, having formed a powerful, brotherly bond with his protector.

Regarding notable abilities, Hiroshi Mochizuki possesses no superhuman powers or combat skills. His primary significance comes from a cherished personal item: the musical pocket watch given to him by his father. The watch's melody is established as a sound that the Neo Organism recognizes from its creation, and this emotional or auditory trigger is powerful enough to disorient the creature during the final confrontation, directly enabling the hero's victory.