Kōji Hiroo, a third-year student at Nohebi Academy, occupies the middle blocker position on the volleyball team, sporting jersey number 5. He measures 185.5 cm tall and maintains straight black hair with bangs obscuring his left eye, creating a stoic, enigmatic presence. Typically lethargic and sparing with words, Hiroo favors taunts or provocative remarks when speaking, aligning with Nohebi's strategy to destabilize opponents psychologically. In Nohebi's Spring High Preliminary match against Nekoma High, Hiroo claims the first point by countering Lev Haiba's block and systematically exploits defensive gaps following libero Yaku's injury-induced substitution. He deploys a precise jump floater serve to pressure Nekoma's adjusted lineup. Actively engaging in coordinated team taunting, he dismisses opponents' successes as luck and openly critiques their errors. Though usually expressionless, Hiroo flares with visible anger when challenged, such as when Kuroo derides Nohebi's sportsmanship. His balanced skill set rates power and jumping at 2/5, technique, speed, and stamina at 3/5, and game sense—his standout trait—at 4/5. As a middle blocker, he contributes offensively with spikes and defensively with blocks, strategically utilizing his jump floater serve to unsettle receivers. Hiroo shares a classroom with captain Suguru Daishō and interacts with teammates in understated cohesion. He remains impassive during motivational speeches, like Numai's rallying address, yet reveals camaraderie through playful third-year wrestling after Nohebi's loss to Nekoma. His name originates from "hiroo umi hebi" (blue-lipped sea krait), fitting Nohebi's snake-themed nomenclature. The "LAND VS. AIR" OVA places Hiroo among Nohebi's team during pre-match activities, accompanying Suguru in confronting Nekoma before the Tokyo semifinals. His specific role in the exchange remains undetailed, though his presence during Tokyo representative matches is confirmed. Documented sources limit Hiroo's portrayal to match appearances, team dynamics, and the noted OVA cameo, without elaboration on backstory, post-graduation, or character development.