Katsuya Kimura attends Domori Elementary School as a student in Class 5-3. He begins as a loner, exhibiting delinquent behavior like smoking and stealing charity donations. His isolation results from feeling misunderstood by classmates, his own cowardice, and a tendency to act worse than he truly feels, particularly toward teacher Nube and peers. Katsuya undergoes significant growth, gradually integrating into the class and becoming one of its core members. Academically challenged, he consistently ranks among the lowest in class grades, alongside Hiroshi Tateno and Makoto Kurita. He displays a pronounced interest in attractive women, often behaving as a self-proclaimed "huge pervert." This fuels a strong infatuation with classmate Miki Hosokawa, who treats him dismissively and likens him to a dog. Beneath his rebellious exterior, Katsuya demonstrates a deeply caring and protective side as an older brother to his younger sister, Manami, contrasting his otherwise troublesome behavior. He appears briefly in flashback sequences involving Nube in the movie "Jigoku Sensei Nube: Gozen 0 toki Nube Shisu" and the spin-off "Reibaishi Izuna: Ascension."