Katsuya Kimura attends class 5-3 at Dōmori Elementary, initially presenting as a delinquent prone to antisocial behavior. Early on, he skips classes, smokes, and steals charity donations, isolating himself due to cowardice, peer misunderstandings, and feelings of inadequacy. Beneath his hardened demeanor lies deep guilt and remorse over these actions. Academically, he ranks near the bottom in non-physical subjects, earning membership in the "Less Than 30 Points Great Trio" with Hiroshi and Makoto. He shines in physical education and home economics, showcasing talent in cooking and sports, while exaggerating his self-professed fishing skills.

His overt attraction to classmate Miki Hosokawa defines much of his conduct. He idolizes her, often enduring manipulation or dismissal, yet this infatuation drives his decisions long-term. They briefly date in high school before parting after one month, though he continues shielding her from supernatural threats.

Guided by teacher Meisuke Nueno and classmates, Kimura evolves significantly, confronting his flaws to become a reliable core member of the group during supernatural conflicts. Despite growth, he retains traces of immaturity and mischief. At home in public housing with his parents, he reveals a devoted, protective side as an older brother to his sister Manami.

In *Jigoku Sensei Nube Neo*, Kimura fails university entrance exams and faces heartbreak after his split with Miki. He perseveres, enrolling at the National Defense Academy as a third-year student to pursue an Army officer role in the Self-Defense Force. This path underscores his enduring resolve to protect others, exemplified when he risks himself to save Miki from spiritual perils post-breakup. Originally a minor character, editorial influence elevated him to a central role. Physically, he stands 156 cm tall and weighs 46 kg, with black hair and eyes, frequently depicted wearing a cap. His hairstyle shifts from long to short as he ages.