Hisashi Ōtomo is a tall high school student with blonde shoulder-length hair, blue eyes, and handsome, prince-like features. His personality is characterized by a socially distant and detached demeanor, leading him to be perceived as a lone wolf who deliberately keeps others at arm's length. This aloofness is rooted in significant childhood trauma. At the age of thirteen, after his parents remarried, his stepsister falsely accused him of inappropriate behavior following an encounter she had initiated, which resulted in his ostracization from the family.

In the aftermath of this event, Hisashi's middle school homeroom teacher, who harbored a secret sexual attraction to him, took him in. This initiated a predatory relationship that continued into his high school years. During this time, Hisashi felt increasingly trapped, questioning the sincerity of the teacher's care while also recognizing the exploitative power imbalance inherent in the relationship.

Upon entering high school, Hisashi became roommates with a student named Mao Tsuchiya. During their first year, a fever-induced delirium caused Hisashi to mistake Mao for his teacher, leading to unwanted physical advances. After regaining clarity, he disclosed his sexuality and his existing relationship with the teacher to Mao. To manage their living situation, he established three rules: Mao would keep Hisashi's sexuality and relationship a secret; Hisashi would not develop romantic feelings for Mao; and both would respect each other's privacy, especially regarding personal time. These rules allowed them to continue rooming together into their second year.

Hisashi's emotional trajectory shifted significantly during the film club's production of a boys' love short film, where he took a lead role, in part, to understand Mao's passion for filmmaking. Shortly after joining the production, he ended the relationship with his teacher, confronting him by openly condemning him as a pedophile during the breakup. This confrontation was fueled by Hisashi's growing realization of the genuine emotional support and stability that Mao provided.

In a vulnerable moment following the breakup, Mao impulsively confessed his feelings for Hisashi. This led to a mutual acknowledgment of romantic attraction and their first consensual physical intimacy, an act that deliberately violated the second rule and initiated a shift toward a committed, secret relationship.

Throughout the film production, Hisashi exhibited increasing emotional openness. He directly expressed love to Mao during a scene and demonstrated protectiveness, such as carrying Mao to the nurse after an injury. His character evolved from indifference to proactive engagement; he joined the drama club to remain within Mao's viewfinder and began articulating future-oriented thoughts about their relationship. He developed a newfound appreciation for hobbies and adorable things like pastel colors, fluffy objects, and penguin-themed items. His favorite places and comforts centered around his dorm room and Mao, indicating a significant shift toward finding safety and belonging within their partnership.