Touko Matsudaira enters Lillian Girls' Academy as a first-year during Yumi Fukuzawa's second year. A distant cousin of Sachiko Ogasawara, she gains the nickname "Mechanical Drill" from Sei Satō due to her distinctive corkscrew pigtails. Initially, Touko exhibits rude and abrasive behavior, especially towards Yumi, criticizing her perceived air-headedness, leniency towards others like Kanako, and naiveté. This attitude creates early tension between Yumi and Sachiko. Secretly, however, Touko admires Yumi, an interest sparked during a prior Valentine's Day card hunt. Orphaned in infancy by a vehicular accident that killed her biological parents, Touko was adopted by the Matsudaira family after no biological relatives could take her in. She later discovered this truth by overhearing relatives and confirming it through a registry. A photograph in her possession shows her biological mother and adoptive mother, who were coincidentally classmates at Lillian. Her biological mother's involvement in the drama club influenced Touko's own participation and contributed to internal conflicts. Profoundly grateful to her adoptive parents, Touko struggles with guilt over feeling unable to repay their kindness, leading her to adopt a "give and take" philosophy on life. This guilt causes her to distance herself, fearing pity and interpreting offers of help, like Yumi's initial rosary, as sympathy. A skilled actress, Touko uses her talent to mask vulnerabilities, effectively hiding complicated relationships within the drama club, even from Sachiko. Despite her cold exterior, moments of vulnerability emerge, such as collapsing in Noriko's arms after realizing excessive harshness towards Yumi. Her emotional barriers start to break during a countryside date with Yumi, visiting her grandfather's hospital. At the accident site, she reveals her tragic past, confessing survivor's guilt, fears of inadequacy, and shows the photograph of her mothers, expressing a desire for Yumi to know her fully. This catharsis allows her to admit her growing fondness for Yumi and that her initial dislike stemmed from envying traits she wished she possessed. Touko's development culminates in accepting Yumi's rosary at the Valentine's Day event, publicly declaring her wish to become Yumi's petite sœur. This follows her realization that fear of rejection made her flee her true feelings. After the event, she formally requests the role privately, leading to their countryside trip and reconciliation. Her journey transforms her from a guarded, manipulative individual into one capable of accepting support, becoming Yumi's official petite sœur and future Rosa Chinensis en bouton. Her relationships include deep-seated admiration for Sachiko that evolves into jealousy over Sachiko's bond with Yumi, and a complex connection to the Matsudaira family, particularly her grandfather, whom she hopes to succeed in managing the family hospital. Noriko provides emotional support during her struggles. Touko's arc emphasizes evolution from isolation to integration, overcoming her fear of vulnerability to embrace the sœur system's support.