Saya's twin sister and the story's main antagonist, Diva was born in 1833.
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.