Sei Satou, known as Rosa Gigantea, is a prominent character with sandy or ochre hair and grey-blue eyes reflecting her partial Caucasian heritage. Her look evolves significantly: longer hair in early years shortens during her third year, culminating in a cropped cut resembling Rei Hasekura's but with full bangs in college. Born December 25, she entered Lillian Girls' Academy from infancy, initially displaying a sullen, rebellious demeanor with little interest in socializing or academics. Her early isolation included a combative childhood friendship with Eriko Torii and a gradually formed bond with Youko Mizuno, who became her trusted confidante.

As a second-year, she became Rosa Gigantea en bouton under her grande sœur, chosen primarily for her striking appearance. This mentorship helped Sei emerge from her shell. A pivotal encounter occurred when she met first-year Shiori in the school chapel. Their intense, clandestine romance ended tragically with Shiori leaving to become a nun, devastating Sei and causing emotional withdrawal. She channeled this pain into writing "The Forest of Briars," an autobiographical novel mirroring the forbidden relationship, which became a discreetly discussed topic at school.

By her third year, Sei transformed into an outgoing, mischievous figure with a playful, teasing persona. She often engaged in physical affection, particularly amusing herself with Yumi Fukuzawa's reactions. Despite her carefree exterior, she demonstrated deep perceptiveness, frequently guiding Yumi on relationships, especially with Sachiko Ogasawara. As Rosa Gigantea, she mentored petite sœur Shimako Toudou. Their relationship appeared distant to outsiders but was rooted in mutual understanding and unspoken empathy; Sei intentionally gave Shimako space to develop independence. She chose Shimako recognizing their shared emotional burdens, though she kept her specific reasons private.

Sei harbored strong dislikes linked to her past: gingko nuts, the school greenhouse (reminding her of Shiori), and Christmas (associated with Shiori's departure). Her interactions revealed protective instincts, like her distrust of Suguru Kashiwagi and mentorship of younger students. After high school, she attended Lillian University, studying English and American Literature. She maintained ties with the Yamayurikai, occasionally visiting to offer advice. Her post-graduation life included driving a yellow Volkswagen New Beetle and nurturing her love for literature, while still grappling with emotional echoes of past relationships. Her dynamic with university classmate Kei Katou, whom she initially mistook during roll call, developed into a close, platonic friendship marked by mutual respect.