Kōshi Usa is a physical education teacher at Aoba High School, serving as the homeroom instructor for Class 3-4 and the advisor for the boys' volleyball club. Born on February 9, with blood type A, he stands at 178 cm tall and is in his late twenties, aging from twenty-seven to twenty-eight over the course of the story. His appearance is characterized by short brown hair and a calm, brownish image color. Despite his outwardly composed look, he has a hot-blooded temperament and tends to react quickly and vocally, sometimes barking out comments like a wild animal. He graduated from the same university as his colleague Kokoro Amatsu, with whom he shares a friendly but competitive relationship, often engaging in teasing rivalry with her. As a teacher, he is dedicated to his students and takes his role seriously, particularly in his efforts to improve the boys' volleyball team, which is relatively weak compared to the girls' team and often the target of jokes from other staff members. Within the larger narrative, Kōshi functions as a supportive adult figure in the school setting, interacting with the main characters from his position as a faculty member. His role primarily involves overseeing school activities, coaching the volleyball club, and providing a grounded, authoritative presence among the younger characters. His development is subtle, centering on his earnest attempts to connect with his students and guide the team toward success, despite his somewhat impulsive nature. Notable abilities include his athletic competence as a physical education teacher and his motivational drive to push the volleyball club to overcome its lack of skill and achieve a first-round victory in competitions.