Nobuyuki Kai is a third-year student at Nekoma High School and vice-captain of the boys' volleyball team, playing as wing spiker with jersey number 2. He stands 176.5 cm tall and weighs 68.9 kg, characterized by jet-black buzzcut hair that grows out slightly post-timeskip, lidded eyes, full lips, and a generally calm, approachable appearance. His demeanor embodies calmness and collectiveness, maintaining a peaceful presence while observing interactions and mediating tensions. He consistently supports team dynamics—such as those between Kuroo and Kenma—without seeking attention, reinforcing cohesion through grounded perspectives during high-pressure situations. Kai joined Nekoma's volleyball club alongside Tetsurō Kuroo and Morisuke Yaku in his first year, sharing their ambition to reach nationals. Despite frequent altercations between teammates, he remained a steady presence throughout their three years. On court, he delivers reliability in receives and digs, with strong technique (4/5) and game sense (4/5). His tactical awareness includes adjusting blocking formations against opponents like Bokuto and executing feints. He mentors younger players, reassuring Lev Haiba during distress by emphasizing team support and offering measured praise to Shōhei Inuoka for improved receives. Strategically subbed in during critical matches like Nekoma vs. Karasuno, he leverages his receiving skills against powerful serves. His final high school match culminates in heartfelt gratitude toward Kuroo and Yaku, highlighting their three-year bond and prompting emotional reactions. Post-graduation, he works as a gardener and arborist. His Okinawan heritage is reflected in his surname (meaning "sea"), darker skin tone, favorite food (sea grapes), and online gaming username "uminchu."