Sumino Kiba is a 14-year-old female student at Akadama Middle School and captain of its Soft Tennis Club. She has white hair, ochre eyes, and a boyish appearance and demeanor. Her personality is hot-headed and competitive, easily provoked and intensely focused on winning tennis matches—especially against her rival Kotone Sawanatsu, whom she initially refused to acknowledge despite their similar temperaments and approaches to the sport.

Their rivalry began in a tennis equipment store where both sought the same racket frame. Sumino exhibits strong self-confidence and deep trust in her tennis abilities, though her overenthusiasm and impulsiveness during matches often cause errors. In serious matches, however, she reveals reliability and strategic calculation, distinguishing her from rivals who lose composure under pressure.

As captain, Sumino leads Akadama Middle School against the primary team from Shiratama Middle School, emphasizing her leadership and commitment to competitive success. Her dynamic with Kotone evolves into a friendly rivalry rooted in their shared fiery determination and occasional errors from excessive zeal. This rivalry drives her appearances and forms the core of her interactions.

The series does not detail her background beyond tennis activities, including family history or external development. Narrative focus remains solely on her athletic role and rivalry, without expansion into spin-offs, movies, or OVAs. She functions as a competitive foil to Kotone, highlighting their parallels while showcasing moments of unexpected competence during matches.