Fiend

A Fiend (魔人) (literally: Jinn or Human Devils) also called Devilman is a devil which has taken over a person's corpse.

Overview
Fiends are human corpses which are taken over by devils as it is their last resort to survive. Devils lose a significant amount of their supernatural power and ability to perform contracts after becoming a fiend. Fiends can be identified by their distinct head shapes, such as Power's horns, Beam's shark head or Pingtsi's hair.

Fiends have the personality of the devils which took over their corpses. However, some fiends retain more of the original corpse's brain than others, leading to their personalities becoming mingled such as Violence Fiend, who should be inclined towards violence, but instead prefers peace. Devils may also deliberately choose to retain half of the original corpse‘s brain to blend into society, as with Asa Mitaka's possession by the War Devil.

With fiends of this variety, both personalities exist separately in one body until the devil decides to takes over the remaining brain or relinquishes full control back to the human. The Devil posessing the human in this case can choose when to control the body, but may not always be able to take over in instances where the human host is under high stress. When not in control, the other personality is still visible and audible to the one controlling the body in the form of a hallucination. Also unique to this type of fiend, the fiend's distinctive head feature will only become visible when the Devil's personality is in control of the body.

Should a fiend die, the devil that possessed the body generally reincarnates once they're in hell. However, since the body they possessed is destroyed, they lose all of their memories of their previous life, and they will develop a new personality should they possess a new corpse.

Abilities
Fiends possess the supernatural abilities of their respective devils but to a lesser degree. Some fiends are able to transform into their original devil forms temporarily. They also can restore their health by consuming blood. However, they are unable to make contracts with devils or humans either.

Trivia

 * The Jinn (جن) is Arabic for "hidden being" and they are either spirits or devils. It is often translated to English as "fiend", "demon", "daemon", "genie", "spirit", and "ghost", and to Japanese as Majin (literally "magical person"). The word "Majin" can also be translated as "human devil".
 * Unnamed fiend is described as Bird-Eating Fiend (鳥を食べていた魔人 Tori o tabete ita majin) in the second Character Popularity Poll.