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Along with her sister Saya, Diva is a Chiropteran queen-- but unlike Saya, she favors Chiropteran dominance over humans.
Origin
In 1833, an ancient mummified body was discovered. At first it seemed like a pregnant human female however up further inspection it was found out to be an unknown species. The scientist thought it to be some form of missing link and were eager to dissect "her". Upon starting the dissection procedure and opened her lower bowel, they were shocked two strange cocoons were discovered by Joel and Amshel Goldschmidt. It was discovered that the cocoons responded to human blood, and they eventually hatched into two twin females: Diva and Saya. In the interest of experimentation, Saya was raised as a human and alongside other humans, while Diva was chained in a jail cell and withheld from any human interaction...until the day that Saya discovered Diva's voice as her unknown twin sang. Saya named Diva, befriended her, and released her from her prison on the day of Joel Goldschmidt's birthday party. That day, Diva set fire to the entire Goldschmidt mansion and murdered Joel and all the party guests while Saya was off collecting a flower to give Joel as a present.
The one exception to her murdering spree was Amshel Goldschmidt, who she made her first Chevalier, or servant. Amshel remained with her throughout the years, but retained his scientist's desire to experiment on her. By his research and business accumen, he spearheaded the various attempts to increase the Chiropteran race to accomplish his queen's dominance, ignoring (or ignorant of) the fact that Diva merely wanted a family, like Saya was able to establish.
As a Chiropteran queen, Diva can only be impregnated by one of Saya's Chevaliers. She was able to accomplish this by kidnapping, presumably raping, and then murdering Saya's young Chevalier (and adopted brother) Riku Miyagusuku. This results both in Diva's ability to take on Riku's physical form, and in her pregnancy-- again, with twin girls. Shortly after their birth (and even while they hatch), Diva dies at the hand of Saya.
Personality
Raised alone and unloved, Diva grows up essentially as a sociopath: she is beautiful and can be charming, but she has no real understanding of the pain and suffering of others, be they humans or her own Chiropteran Chevaliers. She is at times loving of Saya, and other times angry with her: she feels that Saya is her family and should act as such, instead of fighting against Diva to protect humans. She often lashes out at Saya's "fake family" with Kai and Riku, but is in fact desperately jealous of Saya's tight connections with those around her, including Hagi. It was mentioned at one point that Diva's Chevaliers intended Hagi to impregnate Diva, but Diva showed little interest in him, preferring Riku-- Saya's "brother." She also offered to make Kai one of her Chevaliers so that he could be of the same species as Saya and therefore increase the probability of a romantic connection, though of course Kai refused.
Until the end, Diva is presented as more of a tragic figure than a truly cruel one; utterly child-like in her self-absorption but wishing to be able to connect with others.