Manami's boyfriend, he presents himself as polite and sweet in large groups. However, when it suits him, his true self emerges – manipulative and abusive. He has a fetish for bondage and maintains a scrapbook of the girls he has raped. Upon discovering this book, Ayumu is molested by him and forced to become his slave. His plans for her are complicated by his abusive father, who beats him to ensure his relationship with Manami continues, both being from wealthy CEO families.
He stalks Ayumu, attempting to make her call him "Master" for blackmail purposes, and even deceives Ayumu's mother into believing he is a good person. It is revealed early on that he did not choose to attend Nishikō willingly; his father made him enroll because Manami, beneficial for the Sako family's reputation, was a student there. Their relationship might have been orchestrated by their fathers, starting back in middle school. Katsumi tries to break up with Manami but is successful only once. They reunite in both the manga and drama, though in the drama, Katsumi is also shown dating their homeroom teacher, Toda-sensei.
In the manga, Katsumi finds himself threatened by Manami's new boyfriend, Akira, who beats him and humiliates him, mirroring his own treatment of Ayumu. Shortly after, he is seen giving money to Akira, presumably under threat. Despite his academic prowess, he places third in class exams due to injuries inflicted by Akira, with Hatori taking first place.
