Kohei Katsuragi attends Tokyo Metropolitan Advanced Nurturing High School, initially placed in Class 1-A. He possesses a tall, well-built physique and a bald appearance resulting from childhood illness. Katsuragi and his twin sister are orphans; this circumstance makes him solely responsible for celebrating her birthday. Before high school, he consistently held positions on his middle school's student council, serving as president or representative. As Class A's representative, Katsuragi demonstrates high intelligence and a serious, rule-abiding nature. He strictly adheres to principles of proper conduct, evidenced when reprimanding a classmate for mocking Ken Sudō and warning against behavior risking class points. Despite his intimidating appearance, he interacts with lower-class students, though motivations sometimes stem from annoyance with disruptions rather than genuine rapport. He reacts with visible anger when reminded of personal failures, such as his Student Council rejection. During key school events, Katsuragi employs cautious strategies. On the deserted island exam, he forms an alliance with Class C leader Kakeru Ryūen, unaware of being deceived. Class A places third due to Kiyotaka Ayanokōji's interference, damaging Katsuragi's standing. Later, during the cruise ship special exam, he devises the "castle gate" strategy: instructing classmates to remain silent during group discussions to prevent betrayals and maintain Class A's point lead. He defends this approach against Honami Ichinose's objections, prioritizing class protection over collaboration. Katsuragi's relationship with Arisu Sakayanagi, Class A's primary leader, evolves significantly. Initially her advisor and faction rival, he defers to her authority after the island exam failure. When his supporter Yahiko Totsuka is expelled under Sakayanagi's orders, Katsuragi becomes subservient, recognizing her superior leadership. Sakayanagi secures his loyalty by threatening further repercussions for rebellion. Personal moments reveal hidden depth, such as purchasing chocolates for his sister's birthday on August 29. He exhibits uncharacteristic indecision during this errand, emphasizing his sensitivity toward others' feelings despite his stern exterior. In his second year, Katsuragi defects to Ryūen's class (formerly Class C) after persuasion. Motivated by revenge against Sakayanagi, he allies with Ryūen to target Class A. Ryūen discloses Ayanokōji's identity as the mastermind "X," leading Katsuragi to confront Ayanokōji directly. He warns that Ayanokōji's abilities will become widely known, ending his pursuit of a normal school life. Katsuragi's character arc concludes with his transfer to Class C, positioning him against Sakayanagi in the battle for Class A dominance during the third year. This transition marks his shift from conservative tactician to active antagonist in the school's hierarchy.