Sentarō Kawabuchi is a high school student with a delinquent reputation, frequently involved in fights. He possesses pale skin, short spiky hazel hair, brown eyes, a muscular build, and distinctive facial markings: an 'X'-shaped scar under his left eye, often paired with an adhesive bandage on his nose or left eyebrow.

Born to a Japanese mother and an American serviceman, Sentarō experienced early abandonment and discrimination. An initial story claimed his mother left him with her family, where his grandmother rejected him due to anti-American sentiment, leading to his upbringing by his uncle and aunt. This narrative was later exposed as false. He was actually abandoned at a Catholic church shortly after birth, with his birthday recorded as Christmas Day. The church priest cared for him until age three. His aunt and uncle then assumed guardianship, presenting it as a familial arrangement to bypass his grandmother's hostility. The exact truth of his parentage remains uncertain, though speculation suggests his father's sister may have been involved with Americans before disappearing. This complex background fuels his struggles with identity and occasional anger.

Outwardly rough and rebellious, Sentarō exhibits a contrasting personality: deeply loyal and protective toward friends. He is spontaneous, values freedom, and seeks new experiences, particularly through jazz music. His empathy drives him to support others, like intervening when Yurika Fukahori is harassed or helping classmate Seiji Matsuoka pursue musical ambitions to aid his family. These traits reflect an extroverted, optimistic nature, marked by struggles with commitment yet profound compassion. His Catholic faith is central, symbolized by a constant rosary; this faith later shapes his life path.

A skilled jazz drummer, Sentarō rejects other genres. His childhood passion for jazz forms the foundation of his bond with Kaoru Nishimi, whom he mentors in improvisation and performance. Their musical collaboration fosters a close friendship, with Sentarō encouraging the classically trained Kaoru to embrace jazz's spontaneity. He regularly practices in the basement of Mukae Records, owned by the family of his childhood friend Ritsuko Mukae.

Sentarō's relationships undergo significant change. He harbors unrequited love for Yurika Fukahori, who eventually elopes with his idolized older-brother figure, Junichi Katsuragi. Ritsuko initially holds romantic feelings for Sentarō, though he remains unaware until Kaoru confronts him. Sentarō apologizes for his obliviousness, but Ritsuko's affections shift to Kaoru over time. His friendship with Kaoru faces strains, including jealousy over Kaoru's perception of Sentarō's family life and conflicts from Sentarō's involvement with Seiji's band. These tensions culminate in a physical fight but resolve through shared musical expression during a school festival performance where their improvisation repairs their bond.

A pivotal incident occurs when Sentarō attempts to run away upon learning of his stepfather's return. Kaoru intervenes, convincing him to stay. Later, while preparing for another festival, Sentarō is involved in a scooter accident that seriously injures his younger sister, Sachiko. This trauma, combined with his existing guilt and search for purpose, leads him to disappear. Years later, he reemerges as a priest-in-training at a remote island church. Reuniting with Kaoru and Ritsuko—now married and expecting a child—Sentarō vows to support them, signifying his commitment to atonement and spiritual devotion. His journey concludes with him embracing priesthood while maintaining a familial bond with Kaoru and Ritsuko.