Yumi Fukuzawa enters Lillian Girls' Academy as an intensely shy, timid, and insecure first-year. Her pivotal encounter occurs near the Virgin Mary statue when Sachiko Ogasawara adjusts her improperly tied scarf, leaving Yumi deeply embarrassed after Tsutako Takeshima photographs the moment. Seeking to avoid playing Cinderella alongside her male cousin and fiancé, Suguru Kashiwagi, Sachiko unexpectedly offers Yumi her rosary to become her "petite soeur" (younger sister). Feeling hurt by the seemingly random selection and lacking genuine connection, Yumi refuses. Undeterred, Sachiko vows to earn her acceptance.

Following Sachiko to the greenhouse during a Cinderella play rehearsal, Yumi learns of Sachiko's arranged engagement and Suguru's lack of romantic feelings. Though Yumi offers to accept the rosary then, Sachiko declines. Only after the play does Sachiko formally present it again, and Yumi accepts, officially becoming Rosa Chinensis' petite soeur. As Sachiko's petite soeur, Yumi battles intense self-doubt and fear of disappointing her, sometimes avoiding Sachiko to evade being a burden, like during Valentine's Day preparations. Her anxieties manifest in tearful moments, such as crying alone in the greenhouse over perceived rejection. Yet Yumi exhibits profound empathy, notably refraining from calling security during Suguru's confrontation, sensing Sachiko's unspoken distress. Her devotion shows in small acts of service—preparing tea, offering massages—and a steadfast determination to provide emotional support.

Yumi's relationship faces strain from jealousy when Sachiko's cousin, Touko Matsudaira, arrives, and later when Sachiko grows distant after her grandmother's death. Yumi learns the truth and reconciles with Sachiko at the funeral. During a summer visit to Sachiko's family home, Yumi encounters subtle hostility from acquaintances. She diffuses tension at a birthday party by singing "Maria-sama no Kokoro," accompanied by Sachiko on piano, winning the family matriarch's approval. Entering her second year as Rosa Chinensis en bouton, Yumi seeks her own petite soeur. She selects Touko, who initially rejects the rosary, fearing pity due to her hidden trauma as an orphan adopted after her biological parents' death. Yumi helps Touko confront her guilt and grief, leading Touko to accept and become her official successor.

Concurrently, Yumi grows more assertive, organizing a meeting with Hanadera Academy's all-male student council but confessing her deception to Sachiko, believing Sachiko should attend willingly. Sachiko acknowledges Yumi's growth in understanding her. Yumi also receives mentorship and frequent teasing hugs from Sei Satou, which sometimes sparks Sachiko's jealousy. Before Sei's graduation, Sei requests a parting kiss, startling Yumi, but later reveals she will attend nearby Lillian University, maintaining their connection. Yumi acts as a confidant for Yoshino Shimazu, advising on handmade Valentine's gifts for Rei Hasekura. Her familial bonds include a younger brother, Yuuki, whom she occasionally worries about. From a reticent freshman with an ordinary background, Yumi's journey transforms her into a respected mentor within the academy's hierarchy.