Diva's Children

Kanade (奏, Kanade) and Hibiki (響, Hibiki), otherwise known as Kanade Otonashi (音無  奏, Otonashi Kanade) and Hibiki Otonashi (音無 響, Otonashi Hibiki), are the twin daughters of Diva and Riku, and are later raised by Kai.

Overview
Kanade and Hibiki were conceived when their mother, Diva, infiltrated the Red Shield's ship and raped Riku.[32] After a year of being pregnant, they are cut out from Diva's stomach by Amshel as two cocoon-like objects very much like where Diva and Saya were first found in.

As shown through her affection, Diva greatly cared and loved both her children even before they were born, for she had always wanted to have a family more than anything else. Before her death, in her dream scene with her children, Diva is depicted gently stroking the cocoons and singing to them.

At the death of their mother, they finally hatch, revealing that they are female twins who bear great resemblance to Saya and Diva. Having killed Diva, Saya prepares to kill them and herself, but is persuaded by Kai and Hagi not to. However, when Saya finally agrees, Amshel recovers and tries to take Diva's children to replace the loss of their mother, but Saya and Kai protect them, escaping while Hagi holds Amshel off long enough for both to apparently perish in the collapsing building.

Four years later, Diva's children visit Saya during her hibernation. As per his promise to Saya, Kai raises them like they are his own children, even trying to get them to call him "Dad".

As they grew up, Hibiki and Kanade felt they were different from everyone. They were also told to never touch each other's blood. They remained ignorant of the reason for years.

Appearance
Kanade and Hibiki have the same countenance except for the color of their eyes, notably very much like the same as their mother's and aunt's. While Kanade inherited the blue eyes of her mother, Hibiki has the same reddish brown eyes like Saya. They are also shown to have the same pixie cut as Saya when shown with Kai at the epilogue of the anime.

Later in Blood# it is revealed that Hibiki has short hair and glasses while Kanade has longer hair. Except this and the color of their eyes, they're identical.

Personality
As shown from the epilogue, both Kanade and Hibiki are shown to be sprightly and cheerful, being raised by Kai whom kept them away from the bloody history of their ancestral lines. They are quite cheeky as depicted when they kept teasing Kai and not calling him "Dad" at all.

In Blood#, their personalities are almost opposite from each other. Other than having strong sister relationship, their personalities can be compared to Saya and Diva (without being evil), Hibiki for example is very neat, organized and is striving to go to a university while Kanade is unorganized and is striving to make music. Until the traumatic events of the novel, they never called Kai "father".

Trivia

 * The name Kanade means "performance", while Hibiki means "echo/sound". This could be a reference to the names of the previous queens as Diva was a performer and Saya's surname Otonashi means "silent".
 * Kanade and Hibiki are not shown nor mentioned in the manga.
 * They have a vital role in the anime since Diva's blood lost its poisonous properties due to conceiving them.
 * They both have the same hair style as Saya in the epilogue. However, in a new novel of Blood#, Hibiki has short hair similar to Saya Otonashi's and has glasses similar to Saya Kisaragi in Blood-C, while Kanade has long hair, but not longer than Diva's.
 * Although their names weren't revealed in the anime, they were confirmed in the light novels. The novels also point that they were given Saya's surname "Otonashi" instead of Miyagusuku.
 * In the original Japanese dub, they were shortly voiced by Eri Kitamura and Akiko Yajima, who voiced Saya and Diva respectively in the series.
 * In Blood#, Hibiki follows her aunt Saya's behavior by turning David Jr into a chevalier solely to save a life. Kanade's blood is stolen by Adam, who ingests it to become a chevalier.