Manabu Naoi attended Nekoma High School seven years before the main story, playing as a setter wearing jersey number 8. Similar to Keishin Ukai of Karasuno High, he was not a starting player during his high school volleyball career. This shared experience led them to jokingly call themselves the "Benchwarmers Club." After graduation, Naoi returned to Nekoma High as a coach under veteran mentor Nekomata. He maintains a competitive but good-natured rapport with Ukai, now Karasuno's coach, frequently exchanging lighthearted taunts about appearances or coaching abilities during matches. Their interactions reflect mutual respect rooted in their similar pasts. As Nekoma's coach, Naoi actively supports the team during games. He served as the main referee for Karasuno and Nekoma’s practice match reunion, observing Ukai’s early coaching efforts. At training camps, such as the one at Shinzen High, he noted Karasuno’s experimental techniques and joined evening social gatherings with other coaches. His responsibilities include player management, like physically assisting libero Yaku off the court after an ankle injury during the Spring High Preliminary against Nohebi Academy and applying ice to reduce swelling. At the Tokyo Nationals, Naoi demonstrated attentiveness to his players' conditions. He expressed concern over setter Kenma Kozume's unusually high engagement during the intense match against Karasuno, noting how Kenma’s heightened focus accelerated his fatigue. Naoi consulted Nekomata about potentially substituting Kenma for rest but deferred to the veteran coach’s judgment. He also acknowledged captain Kuroo Tetsurou’s leadership in calming teammates like Lev after setbacks. In the OVA "Land vs. Air," depicting Nekoma’s semifinal against Fukurōdani Academy at the Spring High Nationals, Naoi appears among the coaching staff. He witnesses the team’s strategic adjustments, including efforts to counter Bokuto Koutarou’s offensive power through defensive coordination and targeted blocking. Naoi exhibits a confident demeanor with a competitive streak, balanced by humor regarding his non-starting past. He displays perceptiveness during matches, analyzing opponents’ tactics and his own players’ physical and mental states. While generally composed, he occasionally shows visible frustration, such as during Nekoma’s match against Sarukawa Technical High when their persistent defensive strategy visibly wore down Kenma.