Hino Kazuki is Mira Munakata's childhood friend and classmate, sharing a long history that includes another friend, Shun Sakurai. Close friends often call him "Kazu." He possesses a friendly and helpful personality, marked by a protective streak toward Mira. This protectiveness leads him to shadow her like a personal bodyguard, constantly watching over her. Kazuki is also capable of direct confrontation, openly criticizing Mira when he perceives her as naive.

Kazuki harbors longstanding romantic feelings for Mira. After years of keeping these emotions hidden, he eventually confesses his love directly to her and explicitly asks to become her romantic partner, marking a significant moment revealing his struggle with unrequited affection.

Within the narrative, Kazuki functions as one of Mira's multiple romantic suitors. His advances contribute to the central conflicts surrounding Mira, alongside other pursuers like Takayuki Utsunomiya. The OVA adaptation leaves the resolution of his romantic pursuit unresolved, offering no definitive closure to his emotional journey.