Kouji Hiroo is a third-year student at Nohebi Academy and the volleyball team's middle blocker, wearing jersey number 5. Standing 185.5 cm tall, he ranks as the team's second-tallest starting player. His appearance features straight, medium-length black hair with short bangs hanging over his left eye, black eyes, and a characteristically stoic expression. Hiroo typically displays a weary demeanor compared to his teammates, speaking infrequently but often taunting opponents during matches. He employs psychological tactics, dismissing an opponent's successful block as a fluke or targeting specific players like Yamamoto to exploit perceived weaknesses. His gameplay includes accurate jump floater serves and strategic spikes aimed at defensive gaps, such as when he noted Nekoma's substitute libero couldn't match the injured Yaku's skill level. He participates in Nohebi's habitual post-point celebrations, forming a circle with teammates and pointing upward. Hiroo adheres to the team's "goody-two-shoes" act during play, like respectfully admitting to a net touch foul despite the team's unsportsmanlike reputation. When opponents criticize Nohebi's tactics, he internally dismisses them as "whining losers" but avoids direct confrontation. His statistics show above-average game sense (rated 4/5) and balanced technical skills: power and jumping at 2/5 each, with technique, speed, and stamina at 3/5. During Nohebi's Spring High Preliminary match against Nekoma, he scored early points by exploiting Lev's inexperienced blocks but later failed to counter Kenma's setter dump. Despite usual indifference to motivational speeches, like teammate Numai urging the team to reach nationals, he emotionally joined third-year teammates in tearfully teasing Suguru Daishō after their loss. Hiroo shares a class with Suguru and interacts casually with teammates, evidenced by playful wrestling post-defeat. His name derives from "hiroo umi hebi" (blue-lipped sea krait), aligning with Nohebi's snake-themed convention, meaning "Lucky Child" (Kōji) and "Wide Tail" (Hiroo). He is the youngest third-year player on the team. Hiroo's character development occurs solely within the Spring High Preliminary Arc of the manga and anime; he does not appear in the "Land vs. Air" OVA or other spin-offs or later media.