Saya's twin sister is 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.
Not only is she a lovely lady and a beautiful vampire to me but she's hands-down one of my favorite characters of all-time. If you were to ask me who my favorite Blood+ character is, my answer without hesitation will be Diva. She is one of the most well-written characters I've ever come across in recent years. She has a charm that ensnared me from the very first episode she appeared in and that still has me thinking about her character long after the series ended. Diva was and is hands-down among the best female characters one will ever come across during their lifetime in anime or any other media.
-- Leon without a doubt loves and adores you, Diva.