Saya's sister

Diva (ディーヴァ Dīvaundefined), Saya's younger twin sister and the main series antagonist. Diva is the greatest sworn enemy of the Red Shield.

Plot
In 1833, Joel Goldschmidt discovered the mummy of an unknown creature that had two cocoons that react to human blood. Diva was born in 1833 from one of these two cocoons. As an experiment, Joel separated the twin girls, raising Saya as if she were his own daughter while Diva, who was given no name, was locked in a tower and given only the basic necessities to live by Joel's assistant, Amshel. In 1863, Saya discovers the girl locked in the tower and names her Diva. Twenty years later, Saya releases Diva from her tower so that she can sing at Joel's birthday party, however Diva murders Joel along with the rest of the people attending the party before fleeing the area. 

By 2005, when Blood+ begins, Diva has five living chevaliers, who head up the Cinq Flèches Group. Due to her upbringing, Diva has a ruthless nature wrapped in an often child-like demeanor. She giggles after killing people, shows little reaction to the death of her own chevaliers, and tends to destroy things when bored. Diva seems to enjoy tormenting Saya and frequently says she wants to kill her.

After meeting Riku, Saya's adopted brother, she drinks most of his blood and Saya is forced to turn him into a chevalier to save his life. After this incident, Diva becomes increasingly interested in Riku, eventually infiltrating the Red Shield headquarters with Karl to find him. She rapes Riku, and kills him by giving him her blood which crystallizes him. A year after this attack, when the Red Shield is able to regroup, Diva has changed her physical appearance to resemble Riku and retains that appearance until her final battle with Saya.  Diva becomes an opera singer in New York, sponsored by the Group. Cinq Flèches has spread the Delta 67 agent, which can turn people into chiropterans, across various populations of the world, including the United States. When heard live, Diva's voice greatly increases the effectiveness of the agent.

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. Diva seems to treasure her babies and caring for the cocoons. At the Metropolitan Opera House, Diva is set to perform a live broadcast which would cause a large portion of the world's population to turn into chiropterans. The Red Shield and Saya are able to stop the broadcast, and the sisters have their final battle while Diva's chevalier Nathan watches nearby with the babies. 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.

Abilities
Diva is a born singer and she was named after this talent by Saya who heard her sing and henceforth called her 'the singing Diva'. Her mesmerizing voice is not only a sheer talent but also a power that controls and awakens chiropterans.