Mao Daidoji is a student at Kisaragi Girls' High School and a member of the school's newly formed baseball team, where she serves as the catcher and bats eighth in the lineup. She is right-handed both at the plate and in the field. Before joining the baseball team, Mao was a member of the school's judo team, a background that gives her the physical strength and imposing stature she is known for. However, her large build stands in stark contrast to her personality: Mao is deeply timid, bashful, and plagued by a lack of self-confidence. Her time on the judo team was marked by cruel teasing from her teammates, who called her names such as Mao The Cow and Big Ox because of her weight, which further fueled her shyness and insecurity.

Mao is scouted for the baseball team by the energetic team manager, Nene Mouri, who recognizes that Mao's strength and size make her uniquely capable of catching the blistering pitches thrown by the team's ace, Ryo Hayakawa. Indeed, Mao is the only person at the school who can successfully catch Ryo's fastballs, a skill that immediately gives her an essential role on the team. Initially uncertain and hesitant, Mao struggles with her self-doubt and the lingering sting of past bullying. It is through her developing relationship with Ryo that she begins to overcome these barriers. Ryo teaches Mao the value of teamwork and encourages her to believe in her own abilities. Over the course of the series, Mao grows more confident and becomes a reliable and beloved teammate. She learns to trust her physical gifts and finds a sense of belonging on the baseball team that she never experienced on the judo squad.

Mao's motivation shifts from simply being recruited to actively wanting to contribute to the team's success. Her role as catcher is pivotal because without her, Ryo's powerful pitching would be difficult to manage. Beyond her technical importance, Mao's journey is one of personal growth: she transforms from a withdrawn, bullied girl into a confident athlete who understands her worth. Her key relationships include her bond with Ryo, who acts as a source of support and inspiration, as well as her friendships with the other members of the team, who accept her without judgment. Mao's notable abilities include her physical strength derived from judo, her capacity to endure and catch high-velocity pitches, and her developing mental fortitude. She bats and throws right-handed, and her imposing physique makes her a formidable presence behind the plate, even if her gentle nature initially conceals that strength. By the end of the story, Mao has become an integral part of the Kisaragi Girls' Baseball squad, demonstrating that true ability is often paired with a kind heart.