Tōko Matsudaira enters Lillian Girls' Academy as a first-year student during Yumi Fukuzawa’s second year. A distant relative of Sachiko Ogasawara, she quickly earns the nickname "Mechanical Drill" from Sei Satō due to her distinctive corkscrew pigtails. As a member of the Drama Club, Tōko displays remarkable acting prowess, which she employs in significant situations such as the Yamayuri Council’s handling of Shimako Tōdō’s family background and during their stage play. Her skill is so refined that she successfully conceals her strained relationships with her drama club seniors, even from Sachiko. Tōko’s arrival introduces tension between Yumi and Sachiko, partly due to her familiar manner toward Sachiko, whom she addresses as "Sachiko Onee-sama," and partly due to her occasionally rude and critical remarks directed at Yumi. Orphaned in infancy after a vehicular accident that killed her parents and left her the sole survivor, Tōko was placed in an orphanage when no biological relatives could take her in. She was later adopted by the Matsudaira family. Tōko discovered the truth of her adoption by overhearing relatives and confirming it through a registry. A photograph revealed that her biological mother and adoptive mother had been classmates at Lillian, with her adoptive mother sporting the same corkscrew pigtails Tōko now wears. Her biological mother’s involvement in the drama club influenced Tōko’s own participation and the conflicts she later faced there. Beneath a confident exterior, Tōko harbors deep guilt and a sense of unworthiness regarding her adoptive family’s kindness, believing she can never repay them. This leads her to distance herself from others, manifesting as defensive pride, cynicism, and aloofness. She initially rejects Yumi’s offer to become her petite sœur, interpreting it as pity rather than genuine care. As events unfold, Tōko’s fragility and internal conflicts become more apparent. She eventually confides her full history and emotions to Yumi during a date, leading to her acceptance of Yumi’s rosary. This acceptance allows Tōko to come to terms with her past and embrace Yumi’s sincerity, resulting in her becoming Yumi’s petite sœur and the future Rosa Chinensis en bouton. Tōko maintains a relationship with her protective cousin, Suguru Kashiwagi, and her classmates Noriko and Kanako. Though often reverting to defensive behavior when vulnerable, she gradually reveals a softer side through her interactions with Yumi and Noriko.