Mayumura Michiru is a central rival character in the second generation of the Major series. She is the younger twin sister of Wataru Mayumura and the daughter of former professional pitcher Ken Mayumura and his wife Shizuka Maeda, who also has a background in baseball. Growing up in a baseball family, Michiru was raised familiar with the sport and developed a deep passion for it from an early age. She is an ardent fan of Goro Shigeno, the protagonist of the original series, and often refers to him as her favorite pitcher, even decorating her room with his posters.

Michiru has a cheerful, energetic, and strong-willed personality. She is outspoken and not afraid to speak her mind, often showing a tomboyish demeanor that contrasts with her cute appearance. Despite her bluntness, she is deeply caring and supportive of her teammates. When the team faces a difficult situation, she is quick to lift their spirits with encouragement, though she can also be stern when necessary—a trait she inherited from her mother. Her twin brother Wataru has a more sensitive and pressure-prone personality, and Michiru often treats him with a dismissive, no-nonsense attitude, though she ultimately looks out for him.

Her primary motivation is to play baseball at a high level. In her little league years, she batted second and primarily served as a pitcher, shortstop, or second baseman. She has excellent control of curveballs and, when facing a tough opponent, relies on her signature left-handed overhand pitch—a technique heavily influenced by Goro Shigeno. In junior high school, she initially joined the Yokohama Senior team but was forced to leave after being told that girls were not considered part of the lineup. This setback led her to form a soft baseball team with former teammates, and she became the ace pitcher and captain of Ichidai Oi Junior High School's team, where she led them to the semifinals and finals of the spring regional tournament.

Michiru's role in the story is that of a competitive rival to Daigo Shigeno, the son of Goro. Their rivalry began in little league and intensified in junior high, where they faced each other in crucial matches. Her relationship with Daigo also carries a romantic undercurrent; during elementary school, Daigo showed clear affection for her, and in junior high, he remains especially attentive to her, creating a love triangle with Mutsuko Sato, who has feelings for Daigo. This dynamic adds a romantic comedy element to the series.

Over the course of the story, Michiru demonstrates significant personal growth. She faces and overcomes gender-based discrimination in baseball, proving her skill and determination. While initially introduced as a rival, she becomes an important ally and friend, often interacting with Daigo and his circle. Her abilities on the field are notable: she is a versatile fielder, a reliable hitter, and a formidable pitcher whose left-handed overhand delivery challenges even strong opponents. Her strong mental fortitude and leadership make her a respected figure among her peers.