Sei Satou attended Lillian Girls' Academy, serving as Rosa Gigantea on the Yamayuri Council. Her appearance shifted from longer hair in her first two years to a shorter cut in her third year; during university, it resembled Rei Hasekura's style but featured full bangs instead of side bangs. Her exceptional beauty initially motivated her grande soeur to select her as a petite soeur, though their relationship specifics remained undefined.

Personality-wise, Sei projected a playful, devil-may-care attitude, teasing peers like Yumi Fukuzawa and Sachiko Ogasawara to provoke reactions—a facade masking deeper emotional struggles. She intensely disliked gingko nuts and the school greenhouse, both linked to painful memories of Shiori, a first-year student with whom she had a romantic relationship ending when Shiori became a nun. This loss fueled enduring bitterness toward religious figures like the Virgin Mary. Her temper flared when Youko Mizuno interfered in her dynamic with Shimako Toudou, though she later developed greater self-control.

Half-Japanese, Sei was withdrawn and socially disengaged during her early years at Lillian. She adopted a carefree demeanor in her third year, bonding with Yamayuri Council members. Her relationship with official petite soeur Shimako appeared distant or complicated to outsiders. Sei deliberately withheld her reasons for choosing Shimako but acknowledged shared emotional parallels, stating Shimako represented her former self. Their interactions highlighted contrasts: Shimako liked gingko nuts while Sei detested them, and Shimako exhibited responsibility whereas Sei avoided academic commitments. Still, Sei trusted Shimako's independence, believing challenges would strengthen her.

Key relationships defined her further. With Youko Mizuno, she formed a close friendship, confiding during her isolation despite initial resistance; Youko developed unrequited feelings Sei chose not to reciprocate. She teased Sachiko by provoking jealousy through affectionate gestures toward Yumi, yet genuinely cared for Sachiko's well-being. Her bond with Yumi involved playful physical affection—hugging, cheek-kissing, and whispering—attributed to finding Yumi "soft and huggable." She guided Yumi through conflicts with Sachiko and comforted her in the greenhouse despite personal aversion.

Unresolved grief over Shiori permeated Sei's life. She authored "The Forest of Briars" under the pseudonym Sei Suga, a semi-autobiographical novel mirroring their relationship that became a tacitly avoided topic at Lillian. She disliked Christmas, associating it with Shiori's departure, and avoided the greenhouse where pivotal moments occurred.

After high school, Sei studied English and American Literature at Lillian University, maintaining ties with the Yamayurikai and occasionally driving them. She distrusted Suguru Kashiwagi, openly disliking him and shielding others from his influence. During quieter moments—like walking with Shimako on Valentine's night—she displayed subtle emotional availability, sharing chocolate, holding Shimako's hand in her coat pocket, and reassuring her about their future.

Her development culminated in vulnerability: privately informing Shimako first about university acceptance, acknowledging the value of Shimako's homemade cake, and expressing reluctance about their impending separation during graduation, revealing a gradual softening of earlier detachment.