|Saya Otonashi, the central character, is a seemingly ordinary teenage girl adopted and living with amnesia. As her memories return, her life is turned upside down. She was born in 1833, survives on the blood of others, and is destined to destroy chiropterans and her twin sister Diva. Aiding her in her quest are her adopted brothers Kai and Riku and her ever-faithful chevalier Hagi. She also is aided by David, her protector from the powerful organization Red Shield.
Diva also has her own helpers, starting with her five powerful chevaliers: Amshel, Solomon, Karl, James and Nathan. They also have their own human allies as part of the Cinq Flèches Group that they operate, including Van Argiano who helps them develop the Delta 67 substance that Cinq Flèches uses to create chiropteran.
Working against both sides are the Schiff, a group of escaped artificial chiropteran created by Diva's group. The Schiff attack both groups, believing that the blood of Saya, Diva, or one of their chevaliers will save them from a fatal and painful disease they call the Thorn.