Seiichirō Sakaki attends Koyo Academy High School as a first-year in class A, sporting dark blue hair and light blue eyes. His appearance attracts female peers, though he remains indifferent to their attention. Known for his cool, silent demeanor, he enforces strict standards for himself and others. This initially fuels distrust toward teammates—especially newcomer Haru Yamato—prompting him to shoulder all responsibilities alone during play rather than relying on others. His self-sufficiency stems from exceptional futsal talent cultivated since junior high, where his rapid rise to fame began. A versatile all-rounder, he excels in any futsal position but primarily operates as a winger.

He lives with his father, mother, younger sister, and an orange pet cat named Tarou, whom he adores; Tarou features in his SNS profile picture. Sakaki favors quiet environments, cycling as a hobby, and meat-based dishes while disliking noisy individuals and carrots. Unusually, he can sleep anywhere. His blood type is B, and his Capricorn zodiac sign aligns with his January 1st birthday.

Interactions with teammates Haru Yamato, Ryu Nagumo, and Taiga Amakado drive his development. Despite initial reluctance, frequent shared free days gradually foster mutual trust. This marks a shift from his solitary approach, leading him to slowly rely on and cooperate with peers during matches and team activities.