Manami's boyfriend. Around large groups of people he acts polite and sweet; but when it suits him to show his real self, that self is manipulative and abusive.
He has a fetish for bondage and keeps a scrapbook of the girls he has raped. When Ayumu comes across this book, he molests her and makes her his slave. However, his evil plans for her are thwarted because he has an abusive father who beats him to ensure his future relationship with Manami (both of their fathers are rich CEO's of famous companies). He stalks Ayumu, tries to make her call him "Master" for blackmail purposes, and even completely tricks Ayumu's mother into thinking he is a nice person. Early in the series, is revealed that he did not enter Nishikō of his own free will--his father had made him enroll because Manami, who was good for the reputation of the Sako name, would be a student there (and while it has not been elaborated upon, the relationship he and Manami began at Tatsuki Middle School might also have been the work of their fathers). Because of this, Katsumi tries to break up with her, and is successful once; while they do get back together in both the manga and the drama, Katsumi in the drama is shown as also explicitly going out with the Homeroom teacher, Toda-sensei (see below). In the manga, it seems the tables have turned on Katsumi as he is now under threat of Manami's new "boyfriend" Akira, who beats Katsumi and leaves him humiliated, much like Katsumi's treatment of Ayumu. Not long after that, he is seen giving money to Akira after receiving, presumably, a threatening phone call. Despite his good grades, he placed third (Hatori came in first) in the class exams due to his injuries.