Katsuyuki is the leader of the K.T. Butai (Katsuyuki Tsumoto Fighting Group), a kind of ‘sub-unit’ of the Oni-Baku gang, consisting of Yasuo, Atsushi, Saegusa, Kitamura and numerous other students. He has a hair-trigger temper, and as such can cause a lot of trouble without thinking about it. He once fought Eikichi, but was beaten and decided to adopt him as a role model. He is loyal to the Oni-Baku and will fight anyone who badmouths them, but his main allegiance is to Eikichi. He gave himself a scar over his right eyebrow to look tougher, but also has a real one on his abdomen from a fight with Eikichi. He has a rivalry with Tamura, and a long-standing grudge against Kenkichi Hino, who is loyal to Ryuji like how he is loyal to Onizuka. During GTO: Shonan 14 Days we see he is working in a junkyard but he still finds time to be in a biker gang along with Tsukai.