Tsubaki is Soushi's younger sister and is half festum due to their mother being assimilated while still pregnant with her. As a result she spent her entire life in the coagula (a tube with luminescent liquid) located in Valkyrie's Cave and serves as the core of the Brunhilde System. Having absolute control over the island, she also experiences pain whenever Tatsumiya suffers damage in battle. When she left the cave she met a girl named Seri who was studying bugs and befriended her, but had to return to the cave in the final episode as her body couldn't survive for more then three months outside the cave.