Touko Matsudaira enters Lillian Girls' Academy as a first-year student during Yumi Fukuzawa’s second year, swiftly becoming her petite sœur after Yumi—the Rosa Chinensis en bouton of the Yamayuri Council—bestows her with a rosary to formalize their bond. Their dynamic oscillates between earnest admiration and subtle tension as Touko grapples with the responsibilities tied to her role.

Her presence disrupts established dynamics within the council, notably clashing with Hosokawa Kanako, whose intense fixation on Yumi sparks recurring friction. These unresolved conflicts ripple through the narrative, deepening interpersonal complexities.

Later arcs trace Touko’s gradual integration into the council’s senior circles, revealing her nuanced position within the academy’s social structure. While her direct involvement in the council’s duties remains peripheral, fleeting glimpses into her internal conflicts underscore her struggles with tradition, belonging, and the silent pressures of upholding the sœur system’s legacy.