Former Rosa Gigantea  

, 先代ロサ・ギガンティア

In the flashbacks of the story, there is no name given to Seis Onee-sama. Instead, she appears in these scenes. It seems that she chose Sei because of her beautiful face and did not want to put any pressure or expectations on her. She didn't force Sei to find a petite sœur or make Shiori one; instead, she encouraged Sei to follow her own path without regrets. In the third novel, Sei describes Seis Onee-sama as someone who is noble and worthy of being called Gigantea. She truly cares for Sei, as shown after Shiori leaves. After this point, Sei began to depend on her even more. Additionally, before graduating from Lillian, she wished for Sei to find a good petite sœur.