Mutsuko Sakura, a childhood classmate of Daigo Shigeno and the narrative's main heroine, possesses medium-length black hair with straight bangs and green eyes. Her baseball journey starts when she joins the Mifune Dolphins little league team with Daigo and Hikaru Sato, initially displaying solid fundamentals as an outfielder and potential as an all-around player in batting and fielding. Her skills evolve markedly, with early indications of exceptional talent blossoming into consistent contact hitting, notable base-running speed proven by legging out triples, and versatile defensive prowess positioning her as a potential five-tool contributor.

Romantic feelings for Daigo persistently shape her character, acknowledged by others like Kaoru and Izumi Shigeno. This sparks visible shyness in interactions with him, such as hesitating to use his given name despite their closeness. Jealousy flares notably toward Michiru Mayumura when Daigo directs noticeable attention or affectionate honorifics at Michiru, triggering hostility during their initial baseball encounters.

Transitioning to Fuurin Junior High, Mutsuko assumes the baseball club's vice-captaincy. Her leadership navigates team dynamics, including an unexpected benching alongside Daigo in the Combined Team's first practice game. A pivotal moment arises against Tsujido Academy when she recognizes the surname "Sakaguchi" from childhood, linking it to Hikaru Sato and confirming his identity through reactions from Daigo and Hikaru. Her childhood ties to the Shigeno family surface during baseball practices at their home, where her feelings for Daigo become discussion material between Kaoru and Izumi.