Shimako Tōdō enters Lillian Girls' Academy as a first-year student, quickly becoming the Rosa Gigantea en bouton before ascending to the full Rosa Gigantea role. Born to a Buddhist priest father and a mother training to be a Catholic nun, her parents married despite their religious differences. Her mother died shortly after her birth, and her ill father entrusted her to his family before passing away. Raised by her grandparents and uncle in their Buddhist temple, Shimako developed a childhood desire to become a Catholic nun, leading her to Lillian. She entered concealing her religious heritage and initially planned to leave school for a convent.

Her reserved, elegant exterior and serene, doll-like presence mask a strong inner resolve and thoughtfulness. Fair-skinned and calm, she privately grapples with feelings of not belonging and fears relationships act as "shackles," stemming from her belief that her Buddhist background conflicts with Lillian's Catholic world, causing her to initially avoid deep connections. Despite her introverted nature, she demonstrates courage, like voicing concerns over Yumi Fukuzawa's forced induction into the sœur system.

Shimako's foundational relationship is with her grande sœur, Sei Satō. They meet beneath cherry blossoms, and Sei's acceptance of Shimako's rosary helps her find belonging. Though Shimako initially sees her Yamayuri Council role only as personal support for Sei, she gradually values the community. After Sei's graduation, Shimako struggles with loneliness and hesitates to seek a petite sœur, doubting she could replicate her bond with Sei. She often visits their meeting place under the cherry tree, reflecting her lingering attachment.

Her development intensifies upon meeting first-year Noriko Nijō. Noriko's acceptance of Shimako's Buddhist background helps Shimako reconcile her dual identities. Though initially reluctant to formalize their relationship, fearing to "fetter" Noriko or abandon the Yamayuri Council, Shimako eventually offers her rosary during a rainstorm, symbolizing her commitment. This act resolves her conflict, and she openly acknowledges Noriko as her petite sœur. Their bond helps Shimako view Lillian not as a "cage" but as a "nest," making her more open and expressive.

Shimako maintains meaningful interactions with others, including Shizuka Kanina, a rival for Sei's affection. Despite initial tension, she affirms mutual respect with Shizuka, recognizing their shared love for Sei. Friendships with Yumi and Yoshino Shimazu further anchor her in the Yamayuri Council, where she evolves from a reluctant participant to a dedicated leader. Her hobbies, like collecting ginkgo nuts and appreciating cherry blossoms, reflect her quiet introspection and connection to nature, influenced by her uncle.