Koji Hiroo is a third-year student at Nohebi Academy and a starting middle blocker on the volleyball team. Standing 185.5 centimeters tall, he is the team's second-tallest starter. Hiroo has straight, medium-length black hair with short bangs covering his left eye, and his black eyes feature hooded lids, usually set in a stoic expression. Personality-wise, Hiroo displays a lethargic demeanor, speaking infrequently compared to his teammates. When he does speak, it is often to taunt opponents or make provocative remarks, aligning with Nohebi Academy's strategy of psychological tactics during matches. His playing statistics show solid all-around ability, with game sense rated highly at 4/5. Power and jumping are rated lower at 2/5 each, while technical skill, stamina, and speed are average to above average. In competition, Hiroo demonstrates proficiency in spiking and utilizes an accurate jump floater serve. He scored the first point against Nekoma High in the Tokyo Qualifiers by countering Lev Haiba's block and participated in Nohebi's distinctive celebratory routine of gathering in a circle and pointing upward. Hiroo actively targets perceived weaknesses in opponents, such as serving aggressively after Nekoma's starting libero Yaku Morisuke was injured and replaced by Shibayama, internally noting the defensive gap this created for Nohebi to exploit. Hiroo engages in taunting during matches, mocking Lev after a successful block by calling it a fluke and declaring he would score through Lev's block again. He also attempts to unsettle Nekoma's ace Yamamoto by verbally dismissing the threat of his line shots. Hiroo commits to Nohebi's "goody-two-shoes" act, admitting to a net foul without protest despite spectator criticism. Regarding team dynamics, Hiroo maintains solid relationships with his Nohebi teammates, particularly fellow third-years. While his expression remains unchanged during motivational speeches from teammates like Numai, he later joins in playfully teasing captain Suguru Daisho about reconciling with his girlfriend after their loss to Nekoma, indicating camaraderie beneath his typically indifferent exterior.