Kazuhiro Hasukawa is the elder brother of the series' protagonist, Kazuya Hasukawa. Following the untimely death of their parents when the brothers were young, Kazuhiro assumed the role of guardian, balancing the responsibility of raising Kazuya with his own academic pursuits at Ryokuto Academy. This experience of premature responsibility and the need to provide stability for his younger brother profoundly shaped his character, forging a deep sense of loyalty and making him a pillar of support in Kazuya's life. He later takes a position as the school nurse at Ryokuto Academy, a role that allows him to remain a quiet, watchful presence in his brother's daily life.

Kazuhiro is defined by a serious but teasing personality. He maintains a casually observant and playful stance toward his younger brother, often engaging in teasing that can be a source of irritation for the more high-strung Kazuya. Beneath this exterior, however, is an intense protectiveness. He possesses a strong big brother instinct and a fierce "Papa Wolf" mentality, becoming extremely vigilant and interventionist whenever his brother faces genuine emotional distress or harm. He is dedicated and unwavering in his familial devotion, even as he consciously tries to avoid being overbearing, recognizing that his protectiveness can sometimes be unhealthy.

The central event that defines his role in the narrative is his marriage to Sumire, who had been Kazuya's private tutor and first love. This union is the primary motivation for Kazuya's decision to leave home and take up residence in the Greenwood dormitory, as the younger brother feels a profound sense of betrayal and watches his idolized image of Kazuhiro crumble. Despite the rift this causes, Kazuhiro never withdraws his support. He serves as a key figure in his brother's life, not as an antagonist but as a steadying, if complicated, presence. He maintains a relationship with the eccentric residents of Greenwood, particularly Mitsuru Ikeda and Shinobu Tezuka, whom he tolerates as informal mentors for his brother. He monitors their influence discreetly and intervenes only when he feels it may threaten Kazuya's well-being. His marriage to Sumire is later shown to be a genuine and loving partnership, quietly dispelling any outside speculation about his motives.

Throughout the story, Kazuhiro's role subtly shifts from that of a surrogate parent to a trusted confidant, always balancing his brother's need for independence with a silent vigilance over his safety. While his past beyond raising Kazuya remains largely unexamined, he proves to be an anchor of approachable authority within the chaotic world of the series, blending reliability with an understated command that grounds its more eccentric elements. His notable ability is not a physical or academic skill, but rather his quiet, behind-the-scenes management of his family's emotional landscape, and his capacity to provide stability and care from his unique position as both family member and school staff. The birth of his daughter, Midori, with Sumire eventually helps his brother begin to heal and move past his childhood feelings.