Shimako Toudou attends Lillian Girls' Academy, recognized for her reserved nature, striking beauty, and fair complexion reminiscent of an antique Western doll. She possesses long, mid-wavy light brown hair and blue-gray eyes. Her family background is complex: born to Norimichi Toudou, a Buddhist monk, and Yurina Toudou, a Catholic nun-in-training. Her parents defied their religious paths to marry, but her mother died less than a month after Shimako's birth. Her father, afflicted by a fatal illness, entrusted her to his parents and younger brother Masafumi before his own death. Shimako was formally adopted by her grandfather, making Masafumi her older brother.

Raised Buddhist, Shimako aspired from childhood to become a Catholic nun. She enrolled at Lillian at her father's suggestion as a compromise to explore this calling, initially planning to leave mid-schooling to join a convent while keeping her family's religious background secret.

Personally quiet and proper, Shimako possesses strong resolve and thoughtfulness. Early in her school life, she demonstrated empathy and courage, such as voicing concerns during a student council meeting about Yumi Fukuzawa being pressured into the sœur ritual. Internally, she grappled with self-doubt, particularly regarding leadership roles like becoming the next Rosa Gigantea. She felt unsuited for influence and was initially motivated primarily by loyalty to her onee-sama, Sei Satō. Their deeply meaningful yet enigmatic relationship was marked by mutual care and a reflective dynamic mirroring Sei's personality. Shimako confronted her fear of losing relationships, admitting during the Valentine's Day arc that bonds sometimes felt like "shackles."

Her development intensified through becoming a grande sœur to Noriko Nijou, whom she met under cherry blossoms, recalling her first encounter with Sei. This relationship catalyzed emotional growth. A pre-trip phone call to Noriko while in Italy shifted Shimako's perception of Lillian from a "cage" to a "nest," symbolizing security and belonging. This realization allowed her to open her heart more freely, smile more often, and engage warmly with others—a change her light-hearted father actively encouraged. By the series' later stages, Shimako balanced serene composure with a quietly passionate side, including a fervent hobby of collecting ginkgo seeds.

Within broader school life, she participated in events like the Sports Festival and a second-year trip to Italy. Interactions there with peers like Yumi and Yoshino illustrated her evolving comfort in social settings. The Italy trip involved searching for former Rosa Gigantea Sei and reinforced Shimako's ties to Noriko, who assured unwavering support during their separation. This dynamic cemented Shimako's acceptance of Lillian as a voluntary homecoming point. Her relationship with Noriko also refined her approach to the sœur system; she adopted contextual naming conventions ("Shimako-san" outside school, "Onee-sama" within) after observing similar practices in Yoshino and Rei's partnership.