Tsubaki Minashiro  

Brunhilde System, 皆城 乙姫

Tsubaki is Soushi's younger sister and has been half-festum since their mother was assimilated while pregnant with her. She spent her entire life inside Valkyries Cave, where she serves as the core of the Brunhilde System. As the ruler of the island, Tsubaki experiences pain whenever Tatsumiya is injured in battle. After leaving the cave, Tsubaki befriended a girl named Seri who studied bugs. However, due to her inability to survive outside the cave for more than three months, she had to return to her home in the final episode.