Diva is a pure-blooded Chiropteran queen and the primary antagonist, born as the younger twin sister of Saya. Her origin dates to 1833, when two cocoons discovered within a Chiropteran mummy were revived by human blood, leading to the birth of the twin sisters. From the very beginning, the two were subjected to a drastic social experiment. Saya was raised with care and affection as if she were a beloved daughter. Diva, in stark contrast, was given no name and locked away in a dark, cramped tower, receiving only the bare minimum needed for survival and treated as nothing more than a laboratory specimen to be observed and studied.

The profound deprivation of Diva's childhood is the primary force shaping her personality. Because she was denied love, freedom, and human connection, she never learned how to form them. As a result, she exhibits a deeply malevolent and sadistic nature wrapped in an often childlike and air-headed demeanor. She can show a friendly, direct, and charming side one moment and then engage in cruel acts of violence the next, even giggling after killing. Her psychological development was stunted by her isolation, leaving her selfish, mentally underdeveloped, and prone to impulsive destruction when bored. Having had her every whim catered to by her servants after her escape, she is also accustomed to getting whatever she wants. This combination of a tragic past and monstrous behavior makes her a complex figure rather than a simple villain.

Diva's primary motivation is a deep, aching desire for love, family, and the acceptance that was always given to her sister. She is intensely jealous of Saya, who was surrounded by friends, protectors, and a loving home. In her mind, she is perpetually asking why everyone cares only for Saya when they look so similar. She does not seek to rule the world or destroy it for its own sake; such ambitions belong more to her Chevalier, Amshel. Diva's actions, including her most horrific ones, are driven by a wish to have what Saya has. She seeks to take away everything Saya loves, not purely out of malice, but out of a desperate, misguided attempt to fill her own emptiness. She ultimately comes to see her unborn children as the one thing that will finally belong to her and give her the family she always wanted.

In the story's grand conflict, Diva acts as the dark mirror to Saya. While Saya fights to protect humanity, Diva is the figurehead for the forces seeking to subjugate or eliminate it, primarily through the Cinq Flèches organization run by her Chevaliers. Her direct role evolves from a distant, singing menace to a personal and cruel tormentor of Saya's family. A pivotal moment is her assault on Riku, Saya's adoptive brother and Chevalier, whom she locates by the sound of his heartbeat. After raping him, she kills him with her crystallizing blood, an act that leaves her pregnant with his children. In the final arc, she uses her power of vocal mimicry to assume Riku's appearance and becomes a celebrated opera singer in New York, using her voice to activate a virus that would transform millions into Chiropterans.

Her key relationships are all defined by her inability to experience genuine love. Her first Chevalier, Amshel, was a researcher who objectified and manipulated her as an experiment. Other Chevaliers like Karl grew frustrated by her inability to return affection, while Solomon eventually betrayed her, becoming attracted to Saya instead. Even a Chevalier like Nathan is portrayed as a detached observer more interested in watching the drama unfold. Diva is surrounded by servants, yet utterly alone. The only relationship that truly matters to her is the one with her sister, which is a toxic mix of love, hatred, and envy. In their final duel, when Saya's blood causes her to crystallize, Diva does not show malice but reaches out in a final, heartbreaking vision of her newborn daughters, expressing a wish for the love that had always eluded her.

In terms of development, Diva undergoes a subtle but significant shift. Initially a chaotic and bored force of destruction, she becomes more focused and driven once she discovers the possibility of becoming a mother. Her actions become less about random amusement and more about securing a future for her children, even if that future is built on the suffering of humanity. Her final moments are her most human, as she frantically reaches for her babies, transforming from an immortal monster into a dying mother desperate to hold her family. Her arc is a tragedy of a person who, because she was never loved, could only destroy, and who finally realizes she wants to create and nurture when it is too late.

Notable abilities mark Diva as a supremely powerful Chiropteran queen. She possesses superhuman strength, speed, and agility, able to move nearly instantaneously and cause massive destruction with physical blows. Her regenerative abilities allow her to heal from most wounds almost instantly. A signature power is her voice; she is a born singer whose mesmerizing song can control, awaken, and attract other Chiropterans. On a global scale, her live singing could increase the effectiveness of a virus that mutates humans to a 100% success rate. She can shape-shift, most notably to perfectly impersonate Riku, and possesses a telepathic connection to other Chiropterans. Her blood is both a tool and a curse; it can heal and transform humans into her loyal Chevaliers, but it is also lethally poisonous to her sister Saya, causing crystallization upon contact. Conversely, Saya's blood is the one thing that can truly kill her. This lethal incompatibility defines their eternal conflict and ultimately leads to her demise.