Keiko Seki serves as the manager of the baseball team at Myōjō High School, a role defined by her reserved nature and quiet dedication to the team's operations. Her initial loyalty to the team is strongly anchored by her older brother, Masato Seki, who is the team's ace pitcher driven by his ambition to reach the national Kōshien tournament. This personal connection is a primary reason for her early involvement with the group. Prior to joining the baseball team, Keiko was associated with the tennis club, where a boarder named Shin Mikimoto held a quiet admiration for her. However, after receiving an apology from Shin regarding his departure from the tennis club, she begins to reflect on where her efforts would be most meaningful, leading to her decision to pivot and become a co-manager of the baseball team alongside Kasumi Kishimoto.

As the story progresses, Keiko's commitment to the baseball team deepens alongside her growing affection for Yūsaku Takasugi, a key player for the team. Her personality is one of subtle encouragement, particularly toward Yūsaku, and her interactions with Kasumi evolve into a relationship marked by mutual respect and collaborative problem-solving. Keiko's influence on the team is nuanced, often contributing to its overall cohesion from a background position. Her responsibilities are largely administrative, but she occasionally reveals glimpses of musical aptitude, though the full scope of this talent remains undefined. Throughout the narrative, Keiko undergoes a notable development, evolving from a figure who stays in the background to an engaged strategist. This personal growth mirrors the baseball team's own struggles and triumphs, as her personal resolve becomes woven into the collective pursuit of ambition and unity.