Diva

Saya's twin sister and the story's main antagonist.

She provides the Delta67 (derived from her blood) which is used to transform people into Chiropterans. When she was born she was kept in an isolated tower as part of an experiment. Because of that she hated Joel, such that when Saya freed her she slaughtered him and the people at his birthday party. Her isolation left her childish and socially inept as well as ignorant of the moral ramifications of her actions.

She rapes Riku and kills him. As a result of her attack on Riku, Diva becomes pregnant. She does not give birth for over a year, when Amshel cuts the cocoons out of her body. The fight ends as Diva and Saya pierce one another with their swords, which each had coated with their own blood. Saya is unharmed by the attack due to Diva's blood having lost its power when she became pregnant. Diva begins to crystallize from the effects of Saya's blood. Her babies emerge from their cocoons and Diva reaches for them, imagining herself with her daughters and smiling at the vision before dying.

It is implied that Diva had long been seeking the unconditional love of a family, similar to Saya's.