Shimako Tōdō is a student at Lillian Girls' Academy, noted for her prim, proper demeanor and striking beauty. Her very fair skin resembles an antique Western doll, complemented by long, mid-wavy light brown hair and blue-gray eyes. Her background is defined by familial tragedy and religious conflict. Born to Norimichi Tōdō, a Buddhist monk-in-training, and Yurina Tōdō, preparing to be a Catholic nun, her parents married secretly despite their differing faiths. Yurina died less than a month after Shimako's birth. Fatally ill, Norimichi entrusted Shimako to his parents before his death; his younger brother Masafumi raised her. Officially adopted by her grandfather, he became her legal father, making Masafumi her older brother. Masafumi honored Norimichi's wish to share his love of the outdoors, fostering Shimako's appreciation for cherry blossoms and her fervent hobby of collecting ginkgo seeds.

From childhood, Shimako aspired to become a Catholic nun, conflicting with her family's Buddhism. In sixth grade, she demanded her father disown her to pursue this path. He instead compromised by enrolling her in Lilian Academy to acclimate her to Catholicism. She planned to leave midway through high school for a convent and concealed her family's religious background at school.

Shimako's personality blends outward quietness and propriety with inner resolve, thoughtfulness, and calmness. She demonstrates courage in voicing concerns for others' feelings, such as questioning Sachiko Ogasawara's public sœur proposal to Yumi Fukuzawa. Despite her strength, she harbors deep-seated uncertainties about relationships. During the Valentine's Day arc, she revealed a fear of losing connections, viewing relationships and belonging as "shackles," stemming from her orphaned upbringing's instability. Her personality mirrors her grande sœur, the outgoing and nonconformist Rosa Gigantea, Sei Satō.

Shimako served as Sei's petite sœur before succeeding her as Rosa Gigantea. Initial reluctance stemmed from self-doubt about leadership abilities and a desire to work primarily for Sei rather than the student council. Her journey involves significant emotional development. In the first two seasons, she is somber, but a pivotal phone conversation with Noriko Nijou before an Italy trip shifts her perspective. She begins to see the school not as a "cage" but as a "nest"—a place of rest and belonging. Noriko's steadfast presence helps Shimako open her heart more freely and smile more often.

Her relationship with Noriko, whom she meets under cherry blossoms just as she first met Sei, becomes central. She asks Noriko to become her petite sœur, solidifying a bond often described as resembling a pair of Western and Japanese dolls. Noriko's influence aids Shimako in overcoming her fear of emotional connections, further supported by her father's encouragement to embrace openness.

Shimako's role in the Yamayuri Council involves navigating complex dynamics, such as when the council decided to expose her family background, utilizing Tōko Matsudaira's acting skills. Her evolution from a reserved, self-doubting girl to a more emotionally accessible and assured Rosa Gigantea underscores her character arc.