Hisashi Ōtomo is a tall high school student with blonde shoulder-length hair, blue eyes, and handsome, prince-like features. He presents a socially distant, detached personality, often perceived as a lone wolf who keeps others at arm's length. This aloofness stems from significant childhood trauma: at age 13, after his parents remarried, his stepsister falsely accused him of inappropriate behavior following an encounter she initiated, resulting in family ostracization. In the aftermath, his middle school homeroom teacher, harboring sexual attraction, took him in secretly, initiating a predatory relationship that persisted into high school. During this time, Hisashi felt increasingly trapped, questioning the teacher's genuine care while recognizing the relationship's exploitative power imbalance.

Upon entering high school, Hisashi became roommates with Mao Tsuchiya. During their first year, a fever-induced delirium led Hisashi to mistake Mao for his teacher, resulting in unwanted physical advances. After regaining clarity, Hisashi disclosed his sexuality and existing relationship to Mao, establishing three rules: Mao would keep Hisashi's sexuality and relationship secret; Hisashi would not develop romantic feelings for Mao; and both would respect each other's privacy, especially 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 partly to understand Mao's passion for filmmaking. Shortly after joining, he ended the relationship with his teacher by openly condemning him as a pedophile during their breakup. This confrontation was fueled by Hisashi's growing realization of Mao's genuine emotional support and stability. In a vulnerable moment post-breakup, Mao impulsively confessed his feelings, leading to mutual acknowledgment of romantic attraction and their first consensual physical intimacy. This deliberately violated their second rule and initiated a shift toward a committed, secret relationship.

Throughout the film production, Hisashi exhibited increasing emotional openness, directly expressing "I love you" to Mao during a scene and demonstrating 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 shift toward finding safety and belonging within their partnership.