Masato Sakai is an eighth-grade student practicing martial arts with his younger brother Yoshito. He sports the standard boys' school uniform, brown eyes, and brown hair extending to his ears. For martial arts sessions, the brothers don distinctive yellow tracksuits inspired by Bruce Lee's attire in *The Game of Death*.

Masato displays a serious, responsible nature, consistently prioritizing Yoshito's welfare as his primary support and mentor. Though Yoshito occasionally bristles at Masato's cautious conflict approach, Masato remains steadfast in guiding and comforting him.

During school, Masato faced reprimand from teacher Tsurusaki for wearing kung fu shoes in class. Tsubomi Hanasaki intervened in his defense, prompting Masato's later gratitude. When Tsubomi sought martial arts training to gain strength, Masato agreed to instruct her. Their sessions covered nunchuck techniques and balance exercises, with Yoshito demanding Tsubomi address him as "Yoshito-senpai" due to his self-assessed seniority in skill.

A pivotal rift emerged when Yoshito witnessed Masato lose a judo match and subsequently evade confronting a teasing classmate. Incensed by this reluctance, Yoshito fled. Desert Apostles villains exploited his emotional turmoil during this absence, capturing him to forge a Desertrian monster. Masato entered the fray to shield his brother, retrieving Yoshito's crystal prison despite sustaining injuries. This act compelled Yoshito to declare Masato his hero, reconciling their bond.