The Fangire are a kaijin species native to Earth as a result of demonic experimentation and mutations performed on humans in ancient times by Deities and Demons, and thus members of the larger group; The 13 Demon Races. As with other members of this group, the fangire feed on the Prana or "life energy" of humans to survive.
Fangire can take on the appearance of a human and in such a state their true nature can only be seen when they're feeding as outlines and colors of their true face can be seen reflecting onto the surface of their human skin. In their true forms, fangire take on an appearance resembling a monstrous stained glass sculpture of a particular type of animal, whether it be moth, snake, or even more often, bats.
A Fangire's true form can often be fairly indicative of their skills and powers, bat fangire fairly commonly being a high ranking among the species.
When multiple fangire souls clump together they become a being known as a sabbat, that often takes on a draconic and simultaneously ornamental sort of appearance, it's fairly common for these massive beasts to take on multiple heads, showing the ravenous multiple shattered wills that make them up.
Fangire have an extraordinarily long longevity, partially sustained so long due to their draining of prana from other races such as humans and Franken. Their true form bodies tend to be cold to the touch, feeling smooth and rigid as if they were made of stained glass while in reality being much more durable than the material they both feel like and resemble in appearance.
The absorbtion of prana enables them to maintain their bodies and souls in such a way that they can seem immortal, although even fangire seem to have an eventual "expiration."
Interestingly, when a fangire's essence is used to heal a human it's not uncommon for that human to become a fangire themselves, often times of the same type that the essence came from; unless some other strong presence is within to guide the transformation in some way.
Due to the way fangire absorb energy, they can often more easily undertake and use transformations and techniques commonly fatal to other races, such as processing the imperial power of Kivats and Sagarcs. Due to this same energy processing, they're able to safely consume Helheim Fruit which serves as a more than adequate substitute for human prana.
Traditionally a four way sort of kingdom, the fangire ruled under the command of a King, a Queen, and their advisors, the Rook and the Bishop. However in recent years, rule has been under the complete command of its King and Rook (Taiga Noboori and his brother Wataru Kurenai respectively). Due to how Wataru guided things in his brief stay as king and work with the Wonderful Blue Sky Organization and Yggsdrasil corporation, they now live more or less integrated with humans. There's the odd few who break the law and attempt draining humans once again, but those tend to be dealt with by the Fangire royalty.