Nobuyuki Kai, a third-year Nekoma High student with black buzzcut hair, dark eyes, and a calm, approachable appearance, serves as vice-captain of the boys' volleyball team. Standing 176.5 cm tall and weighing 68.9 kg, he wears jersey number 2. His personality reflects calmness and collectiveness; he often remains in the background but speaks politely when engaging. Kai joined the Nekoma volleyball club alongside Tetsurō Kuroo and Morisuke Yaku in their first year, united by the goal of reaching nationals. During the Tokyo Prefectural Qualifiers, Kai supports Taketora Yamamoto through embarrassment over his sister Akane's cheering against Fukurōdani Academy, contributing receives and spikes. He also strategizes with Kenma Kozume regarding Bokuto Koutarou's habits. When Yaku injures his ankle versus Nohebi Academy, Kai maintains focus, later identifying Nohebi's targeting of Yamamoto and successfully receiving Takachiho's serve to counter their tactics. He celebrates Nekoma's victory with Kuroo and Yaku. In the Land VS Air OVA, Kai plays for Tokyo's representative against Fukurōdani at Spring High Nationals. He scores an early point with a spike and receives Bokuto's powerful serve. During the match, he supports Lev Haiba after mistakes and acknowledges Akane Yamamoto's admiration for Lev's jumping height. At Tokyo Nationals, Kai delivers solid receives against Kiyokawa's ace and detects Sarukawa's feints, earning announcer praise. The night before the Sarukawa match, he encourages Yaku to trust junior teammates for the following year and confirms Kenma's heightened determination after meeting Karasuno's Hinata Shoyo. Tasked with receiving Tobio Kageyama's powerful serves against Karasuno, Kai is subbed out for Inuoka late in the second set to strengthen blocking against Hinata, returning for the third set. Post-match, his expression of gratitude for three years playing with Kuroo and Yaku moves them to tears. Later, he observes Hoshiumi Korai's exceptional receiving during Karasuno's match against Kamomedai. Post-high school, Kai becomes a gardener and arborist. He reunites with former teammates at Kenma's house during a later gathering. A one-shot manga titled "Nobuyuki Kai's Nightmare" exists, though sources lack specific content details. His Okinawan heritage is suggested through his surname meaning "sea," his darker skin tone, and his favorite food being Okinawan sea grapes, though not explicitly confirmed. Kai maintains strong bonds with teammates, particularly his three-year connection with Kuroo and Yaku. He supports underclassmen like Lev and Inuoka during struggles. His family includes younger sister Akane, who cheers for Nekoma at matches. His volleyball statistics show balanced abilities: power 3/5, jumping 2/5, stamina 3/5, game sense 4/5, technique 4/5, and speed 3/5. Kai's playing style emphasizes defensive stability, consistent receives, and tactical awareness, aligning with Nekoma's "blood" philosophy of sustaining rallies. He contributes offensively with occasional spikes and serves.