The character known as Former Rosa Gigantea held the position of White Rose within Lillian Girls' Academy's Yamayuri Council before the main story. As one of the student council's three highest-ranking members, she oversaw the sœur system pairing older mentors with younger students.

Her most significant bond was with Sei Satō, whom she chose as her petite sœur. While initially claiming selection was simply because she "liked her face," she later revealed this reason was insincere, admitting she concealed her true motivation to avoid burdening Sei.

Before graduation, she shared vital counsel with Sei, warning against becoming overly absorbed in treasured individuals. She further advised Sei to extend similar support and kindness to her own future petite sœur, embodying the mentorship cycle's continuity within the academy's traditions.