Kunimitsu Tezuka captains the Seishun Academy (Seigaku) tennis team in his third year. He has dark brown hair, wears glasses, stands 179 cm tall, and weighs 58 kg. His blood type is O, and his unaided eyesight measures 1.5 in both eyes. Tezuka consistently presents a stoic expression and is often mistaken for a teacher due to his mature demeanor, a trait influenced by his grandfather. Tezuka enrolled at Seishun Academy three years before the main story. His exceptional talent as a first-year provoked jealousy among upperclassmen, leading to a deliberate injury of his left arm during practice. This resulted in chronic pain and forced him to seal his signature technique, Hyaku Ren Jitoku no Kiwami (Pinnacle of Hard Work), which heavily relied on his left arm. Though he initially quit the club over ethical objections to violence in tennis, then-captain Yūdai Yamato persuaded him to stay and become Seigaku’s "Pillar of Support," entrusting him with the goal of reaching the Nationals. During this period, he also formed a promise with teammate Shuichiro Oishi to lead the team to the national tournament. As captain, Tezuka imposes strict discipline, frequently punishing rule violations with collective laps. His leadership emphasizes self-reliance and growth, notably mentoring protagonist Ryoma Echizen to eventually succeed him as Seigaku’s pillar. Recognizing Ryoma’s potential early, he allowed him to join the regulars and later challenged him to unofficial matches to accelerate his development. Tezuka’s dedication to the team often overrides personal health; he consistently risks aggravating his arm injury during critical matches, such as against Hyotei Academy’s Keigo Atobe in the Tokyo Prefectural Tournament. This match ended in defeat after his arm collapsed, prompting him to seek treatment in Kyūshū (Germany in the anime). During rehabilitation, he confronted psychological trauma ("yips") that initially hindered his ability to raise his arm overhead. Tezuka commands an all-rounder playing style, adaptable to various court positions. His dominant left arm executes advanced techniques including the Zero-Shiki Drop Shot, a drop shot rolling toward the net without bouncing; the Tezuka Zone, which imparts spin to draw shots toward him; and the Tezuka Phantom, a reverse application of Tezuka Zone forcing shots out of bounds, though its excessive use risks arm injury. He also wields Hyaku Ren Jitoku no Kiwami, concentrating the "Muga no Kyouchi" (State of Self-Actualization) into his left arm to double return strength/spin while reducing stamina loss, and Saiki Kanpatsu no Kiwami (Pinnacle of Great Wisdom), predicting match outcomes through mental simulation. He later masters "Teni Muhou no Kiwami" (Pinnacle of Heavenly Endowment), fully unlocking his potential during the U-17 World Cup. After rehabilitation, Tezuka led Seigaku to victory in the National Tournament. Key matches included defeating Higa’s Kite Eishiro to reinforce ethical play, overwhelming Hyotei’s Kabaji Munehiro in rainy conditions, and dominating Shitenhoji’s Chitose Senri 6-1 using his new Zero-Shiki Serve. In the finals against Rikkai Dai’s Sanada Genichiro, he unveiled Tezuka Phantom but ultimately lost due to accumulated arm strain. Following Seigaku’s national title, he departed for Germany to pursue a professional career. During the U-17 World Cup, Tezuka represented Germany after declining further participation in the Japanese camp. He defeated Atobe Keigo and Inui Sadaharu in a doubles match alongside Q.P., showcasing evolved skills. Later, he faced Seigaku teammate Fuji Shusuke in the semifinals, securing a three-set victory. His most significant U-17 match was against Rikkai captain Yukimura Seiichi in the semifinals, where he prevailed in straight sets. Tezuka maintains rigorous daily routines: rising at 5:30 AM for training, studying English via radio, and excelling academically, particularly in World History. His hobbies include mountaineering, fishing, and woodworking, often influenced by his grandfather. He prefers traditional Japanese cuisine like unagi cha (eel rice) and consumes classical literature and films. His room features organized bookshelves, fishing gear, and photographs of mountains he has climbed. Relationships central to his development include Fuji Shusuke, a close friend and rival since their first year whose matches underscore mutual respect and unresolved competition; Echizen Ryoma, viewed as both a protégé and successor, mirroring Tezuka’s earlier dynamic with Captain Yamato; Kunikazu Tezuka (Grandfather), a significant influence on his discipline and values, sharing activities like fishing; and Chitose Senri and Miyuki, bonding with Senri’s sister Miyuki during rehabilitation where he aided her tennis confidence while overcoming his own trauma.