Kunimitsu Tezuka is a third-year student at Seishun Academy Junior High School (Seigaku) and the captain of its tennis team. He is widely regarded as a national-level player and one of the most formidable competitors of his generation. Tezuka stands at 179 centimeters, has dark brown hair and brown eyes, wears glasses, and is nearly always seen with a calm, stoic expression.

His tennis career began years before the main storyline. As a child, he demonstrated exceptional talent, easily defeating the runner-up of a junior tennis tournament, Genichiro Sanada, and playing evenly against champion Seiichi Yukimura. Upon entering Seishun Academy, his skills already surpassed most upperclassmen, but his reserved, serious attitude and the fact that he played with his non-dominant right hand while being a natural left-hander led teammates to view him as arrogant. A jealous upperclassman injured Tezuka‘s left arm, causing chronic pain that would plague him throughout the story. Disgusted by the use of a racket to harm others, Tezuka initially decided to leave the club, but then-captain Yudai Yamato convinced him to stay and become the team’s “pillar of support.” A private match against fellow prodigy Shusuke Fuji ended in Tezuka’s defeat while his arm was still injured; Fuji, unhappy to learn of the injury, promised a rematch. Despite his arm problems, Tezuka secured a regular spot in his first year, became vice-captain in his second year, and later captain.

Tezuka’s personality is defined by unwavering seriousness, self-discipline, and fairness. He is strict in enforcing team rules—often making the entire team run laps for infractions—and is known for the phrase “Yudan sezu ni ikō” (Don’t let your guard down). He rarely shows visible emotion, maintaining a stoic front even in victory or when confronted with bizarre situations, such as drinking his teammate Sadaharu Inui’s notorious vegetable juice without a visible reaction. Beneath this exterior, he cares deeply about the team’s success and the growth of its members. He is willing to sacrifice his own health, repeatedly ignoring medical advice and risking permanent damage to his left arm in order to defeat opponents or inspire his team. This self-sacrificing nature is central to his role: he considers the team’s national tournament goal more important than his personal well-being.

His primary motivation is to lead Seigaku to the National Tennis Tournament and to prepare the next generation. He identifies immense potential in the first-year Ryoma Echizen and takes on a mentoring role. In an unofficial match, he decisively defeats Ryoma to push him to evolve and tells him to become Seigaku’s new pillar of support. Ryoma consequently views Tezuka as both a mentor and rival. Tezuka often stays on the sidelines during early tournament matches because the team wins before his singles match is needed, but his true ability is shown when he steps onto the court.

His notable abilities include an all-rounder play style and a left-handed grip. His signature techniques are the Zero‑Shiki Drop Shot (a drop shot that lands and rolls back toward the net without bouncing), the Zero‑Shiki Serve, the Tezuka Zone (a spin-based field that draws all opponents’ shots toward him), and the Tezuka Phantom (an inverted version that forces balls out of bounds). He also unlocks the Selfless State (Muga no Kyouchi) and its three advanced “Perfection” stages: the Pinnacle of Hard‑Won Mastery (Hyaku Ren Jitoku no Kiwami), which concentrates power into his arm; the Pinnacle of Brilliant Insight (Saikikanpatsu no Kiwami), granting absolute prediction; and the Pinnacle of Harmony (also called the Light of Pride), achieved through pure enjoyment of tennis, which unleashes full potential while overwhelming opponents.

Key relationships shape his journey. Shusuke Fuji is both a close friend and a quiet rival; they have shared the team since their first year and Fuji has long awaited a full-strength rematch. Shuichiro Oishi, the vice‑captain, is a trusted friend who understands Tezuka’s burdens. With Ryoma Echizen, Tezuka has a mentor–rival dynamic, deliberately challenging him to foster growth. His rivalries with other school captains are intense and respectful: Keigo Atobe of Hyotei, known as the “King,” pushes Tezuka into a grueling match that re‑injures his arm; Genichiro Sanada of Rikkai, the “Emperor,” whom Tezuka defeated years earlier; and Seiichi Yukimura, the champion from that early encounter. Family includes his father Kuniharu, a trading company employee; his mother Ayana, the only one who can detect a faint smile on his face; and his grandfather Kunikazu, a police officer and judo instructor, who heavily influenced Tezuka’s strict, upright character.

Tezuka undergoes notable development around his injury and psychological barriers. The pain from his match with Atobe becomes a traumatic memory that prevents him from lifting his arm over his shoulder. During a medical retreat in Kyushu, he gradually overcomes this fear and regains full motion. He also learns to balance his self‑sacrificial tendencies with the need to remain a lasting pillar for the team. While his outward demeanor remains largely unchanging, moments of subtle warmth emerge—he smiles briefly when told his arm is healing and again, for an instant, after winning the Nationals.

Tezuka’s combination of extraordinary ability, steadfast leadership, and quiet mentorship has made him a respected and popular figure within the story. He is often seen as older than his actual age due to his mature composure, yet he retains a hidden fear of horror stories dating back to a panic in the school library as a first-year. Through all his matches and trials, he remains defined by his determination to protect his team and help those around him surpass their limits.