Masako is a high school student who initially appears as a mild-mannered, academically above-average girl who is well liked by her peers and also active as a cyclist athlete. Despite her generally positive social standing and athletic build, she harbors deep insecurity about her small chest size. This insecurity is triggered when she confesses her feelings to a popular boy named Bando, who rejects her outright because he dislikes flat-chested women. The rejection sends Masako into a spiral of resentment, and she decides to become a vigilante avenger for all women with small chests. She adopts the alias the Big Boobs Buster, a masked persona that targets female classmates who have large breasts. Using a pervert’s stolen list of well-endowed girls, she subdues her victims and makes spray casts of their chests, then blackmails them with the threat of exposure should they ever date Bando or any other boy at school. Her motivation is driven by a mix of personal humiliation and a misguided sense of justice, believing that by humiliating larger-breasted girls she can redress the imbalance caused by body shaming.

Masako’s role in the story is that of the protagonist whose misguided crusade gradually leads her to confront her own body image issues. Her key relationship is with Kyoko Mitoizumi, a track and field competitor with a large chest who discovers Masako’s true identity. Instead of exposing her, Kyoko blackmails Masako into substituting for her in a sprint race due to an injury. During the intense training that follows, Masako begins to develop genuine feelings of friendship toward Kyoko and starts questioning the morality and purpose of her campaign. This relationship serves as a catalyst for her character development, shifting her focus from vengeance to self-acceptance. Her parents appear in the story but play a peripheral role that does not significantly influence her arc.

Masako’s notable abilities include physical fitness as a cyclist and sprinter, which she uses during her vigilante activities and later in athletic competition. She also demonstrates resourcefulness in creating and executing her blackmail scheme. Over the course of the narrative, Masako evolves from a scorned, revenge-driven teenager into someone who begins to embrace body positivity and reevaluates her destructive behavior, ultimately moving toward friendship and personal growth.