Miyuki Ōnuma is a supporting character in the volleyball sports story Attack No. 1. She first appears in the high school arc as the third-year captain of the Fujimi High School girls' volleyball team. From a wealthy family, she initially comes across as a strong-willed, demanding, and somewhat selfish leader. When the protagonist Kozue Ayuhara and her friend Midori Hayakawa join the high school team, Ōnuma and her close followers try to drive them away through dirty tactics and harassment. In the anime adaptation, her followers even resort to putting a harmful substance on Kozue's towel to impair her vision. However, after a humiliating loss to the Misawa Gakuin team, Ōnuma's followers quit, and she is left alone. It is Kozue's persuasion and belief in her that convinces Ōnuma to commit herself seriously to volleyball. From that turning point, she sheds her antagonistic role and becomes a reliable teammate and eventually a close friend to Kozue and Midori. Her leadership skills, which had previously manifested as bossiness, are redirected into positive captaincy. She is a well-rounded player with particular strength in attacking and is known for developing combination techniques with Kozue, such as the double attack and later the double spin attack. Her defensive abilities, especially in serving, are also notable, and she is considered one of the team's most powerful players alongside Kozue. At the end of her third year, Ōnuma passes the captaincy to Kozue and retires from the school club, having grown from a rival into a respected mentor and friend. Her key relationships include her growing mutual respect and friendship with Kozue, a competitive but ultimately supportive bond with Midori, and her role as a unifying force for the younger members of the team. Throughout the high school portion of the story, Ōnuma undergoes clear character development, transforming from a self-centered, power-hungry captain into a dedicated and team-oriented player who values hard work and camaraderie.