Chiropteran

Chiropterans are hematophagus bat-like creatures that live by feeding on human blood.

Origin
Chiropterans are created from the blood of a queen. So far in the anime, Diva's blood is used to create most of the chiropterans. Once a human is injected with the blood, he/she turns into a chiropteran gradually. The process can be sped up with using enhancements or Diva's voice.

Characteristics
They generally possess extraordinary speed, strength, and supernatural healing abilities. They are also capable of contorting and transforming various body parts in order to fly or have longer limbs. Higher level chiropterans have the ability to disguise themselves as ordinary humans.

Chiropterans can heal almost instantly from most wounds. So, bullets are non-lethal and only useful in slowing them down. Even explosives do not seem to be capable of killing them, though if buried or sealed inside stone they will go into hibernation until freed.

Process of Elimination
Ironically, the blood of queen is also the primary method of killing them. When the blood of another queen is introduced into a chiropteran's bloodstream, it starts a crystallizing chain-reaction that destroys them. Thus, chiropterans created from Diva's blood can be killed by Saya's blood, and vice versa.