Manabu Naoi coaches the Nekoma High School volleyball team. At 26 years old, he has a sturdy, muscular build, a rectangular head shape, a prominent jawline, short dark hair, and dark eyes, maintaining a consistent appearance. Naoi attended Nekoma High seven years before becoming coach, playing as a setter wearing jersey number 8. He was not a starting member during his high school career. This shared experience created a connection with Karasuno High's coach, Keishin Ukai, another non-starting setter from the same era. They humorously dubbed themselves the "Benchwarmers Club" due to their frequent time on the bench. As a coach, Naoi balances encouragement with strictness towards his team. He maintains a competitive yet respectful dynamic with Ukai, exchanging lighthearted taunts about appearances or coaching abilities during interactions, like when their teams faced off in a practice match where Naoi served as main referee. Together, they challenged each other to future victories. His coaching includes training camps, such as one at Shinzen High where he initially mistook Karasuno's experimental tactics for poor performance before recognizing their intent to improve. During tournaments, Naoi actively supports the team from the sidelines. He expressed frustration when opponents like Sarukawa Tech used defensive strategies to exhaust Nekoma's setter, Kenma. Conversely, he showed surprise and concern when Kenma displayed unusual enthusiasm and exertion, especially against Karasuno, noting the physical toll. He suggested substitutions for Kenma's rest but deferred to head coach Nekomata's decisions. In a match against Nohebi Academy, Naoi assisted carrying the injured player Yaku off the court and applied ice to his ankle. Off the court, Naoi often socializes with other coaches, frequently being dragged out for drinks after practices. Despite his rugged appearance suggesting alcohol tolerance, he is known to be a lightweight drinker. His name reflects meaning: "Manabu" (学) translates to "learn," while "Naoi" (直井) combines "straight" (直) and "well" (井).