Chiropteran

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

Origin
Regular chiropterans can be created from the blood of a queen. So far in the anime, Diva's blood is used to create most of the chiropterans. However, unlike the process of creating chevaliers, creating chiropterans takes indirect injection of queen's blood. The blood is first taken from the queen. This is either injected into live human bodies or manipulated into delta particles which are used in all cinq fleches products. The consumption of the products lead to gradual transformation into a chiropteran. The process can be sped up with using enhancement like Diva's voice. Partial transformation can take place as well, e.g. in case of Mui. Sometimes, receiving scar injury from chiropterans might also lead to partial transformation, e.g. in case of George (however, it's unknown if the infection was due to medical treatment given to him after he was kidnapped to Yanbaru facility).

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. Another way to eliminate a chiropteran is by the use of decapitation even though it is believed not to be a viable method for humans but it does give a much higher chance of success than the use of bullets. As well blood loss is another method without blood the Chiropteran's strength, speed and healing capabilties are greatly diminishes allowing bullets to be more effective than perviously believed.