Kousuke Ohshiba is a central figure in the narrative, introduced as a math teacher in his late twenties who stands at a tall and lean 183 centimeters. His physical presence is marked by black hair, blue-gray eyes, and a small mole on his chin, giving him a composed yet approachable appearance. Behind his professional classroom demeanor lies a formidable reputation from his earlier years: he was once known on the streets as the “Bear Killer,” a lone fighter who systematically broke up local gangs that preyed on the vulnerable. This background informs much of his protective instinct and his readiness to confront threats, but it also stands in contrast to the calm, nurturing role he adopts as an educator.

His personality blends gentle warmth with a subtle, teasing sense of humor. He genuinely cares for the well-being of those around him, particularly younger individuals who are struggling with their circumstances. This compassion is paired with a quietly authoritative side, allowing him to offer firm guidance without being harsh. He can be uncompromising when it comes to protecting someone he cares about and often places their needs ahead of his own, which at times borders on being overprotective. Underneath this outward steadiness, he wrestles with his own emotional complexity and a lingering uncertainty about how to reconcile his desires with his responsibilities.

Kousuke’s primary motivation is to ensure the happiness and safety of the people he has taken under his wing. He is driven by a sincere wish to see them overcome their difficulties and find a stable, fulfilling life. This drive originally surfaces as a mentor’s duty but grows into a deeply personal commitment that pushes him to reevaluate his own boundaries and principles. He does not seek recognition or heroism; rather, he acts out of a fundamental refusal to let anyone endure the kind of isolation and helplessness he has witnessed.

Within the story, Kousuke serves as both a professional educator and a personal anchor for the main protagonist, Masahiro Setagawa. When he first meets Masahiro, a bullied teenager trapped in gang exploitation, he steps in decisively, not only as a rescuer but also as someone who offers a sense of belonging. His classroom becomes a space of safety, and his home eventually a genuine refuge. As the narrative progresses, his role shifts from distant protector to involved partner, blurring the line between teacher and guardian and forcing him to confront what that means for his career and his conscience.

His most significant relationship is with Masahiro Setagawa, his student, mentee, live-in charge, and eventually his fiancé. The bond is built on a foundation of trust and unconditional protection, though it carries the inherent tension of an authority figure entering a romantic partnership with someone younger and formerly under his care. Kousuke extends a similar protective affection to his younger brother, Kensuke, with whom he maintains a steady, supportive sibling dynamic. He also shares a reliable friendship with Asaya Hasekura, a peer who understands his mature outlook and offers camaraderie outside his immediate household. Against those who harass or endanger his loved ones, Kousuke harbors an unyielding antagonism, and his old street-fighter edge can resurface when such lines are crossed.

Over the course of the series, Kousuke undergoes a notable shift. He begins as a somewhat passive guardian who provides safety and instruction from a measured distance. Key moments force him to own his personal feelings instead of hiding behind duty, and by a turning point around the mid-point of the story, he takes active steps to claim his romantic future rather than simply safeguarding Masahiro’s. This development sees him learning to balance the ethical weight of his position with a life built on genuine mutual affection, moving from a self-appointed hero into a fully realized partner.

While Kousuke possesses no supernatural abilities, his strengths are grounded in his sharp analytical mind and high emotional intelligence. As a mathematics teacher, he demonstrates strong problem-solving skills and a structured way of thinking. More importantly, he excels in conflict resolution and creating an environment where difficult truths can be spoken. His past as a fighter also leaves him with disciplined physical competence, though he relies far more on communication and empathy than on force. These qualities together make him an effective protector and a patient, perceptive presence in the lives he touches.