Tsubaki is Soushi's younger sister and the product of their mother's assimilation while pregnant—making her half Festum. She spent her entire life suspended in the coagula, a luminescent tank housed in Valkyrie's Cave, where she essentially serves as the living core of the Brunhilde System. This gives her absolute control over the island, though it comes at a cost: she experiences physical pain whenever Tatsumiya takes damage in combat.
Her only real taste of freedom came when she briefly left the cave and befriended a girl named Seri, who was researching insects. The connection was meaningful, but it couldn't last—her body simply couldn't survive outside the coagula for more than three months. By the series' end, she had to return to her tank, accepting her role as the island's anchor.
Content compiled by AnimeList.moe from publicly available sources.
