Fiend

A Fiend (魔人), 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 possessing 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.

"Who" is the fiend
There has been some misconception over "who" is the fiend. Excluding rare cases such as Asa Mitaka and Yoru who have a very unique relationship when it comes to fiends, the fiend is not the person being possessed but the devil themselves taking over the person. If one were to talk to a fiend they would be talking to the devil and not the human. There have been statements by some fans that fiend is the host itself with a devil inside them but this is incorrect and could be caused by confusing them with Hybrids. They aren't a newly born entity who was the result of the fusion between a human and devil either as when it comes to a situation like Galgali's; he was the Violence Devil in the body of the human but had absorbed so much of the human's memories that they seemed very real to him but he was not a newly created being as the result of the fusion, nor was he truly the actual human brought back to life, but a possessor who absorbed many memories of one he possessed and had their personality altered. To put it simply; the fiend is the devil puppeteering a human corpse. If it is a situation like the Violence Fiend, then it just means the devil has had their memories and personality altered but it is not a true resurrection of the human themselves nor is it a newly created person. If it is a situation like the War Fiend, the human and devil's true personalities can co-exist but, as of now, seems like only the devil has complete control of who gets to remain alive and can take away the human's consciousness at any time.

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.

Power had the ability to revive as a devil from her pact of blood consumption with the Chainsaw Man. However it could be that because she is the Blood Fiend/Devil it could be unique to her that this blood pact with a powerful enough devil/hybrid was enough to fully revive her as a standard devil again. Yoru made the claim that if Asa were to help in her accomplishing her goal that she promises to give her body back. While this could have been just some way of persuading her to follow her orders, it is possible that War Devil has the rare ability of freely leaving out of the state of being a fiend without having to die and then reincarnate. Note that War Devil was in a severely weakened state even as a full Devil so if it is true, then perhaps other devils with an ability to de-fiend themselves can do it even when very weak or it could only apply to weakened Devils of certain types or a specific status. It is unknown if other fiends have a special ability or their own individual way of returning to their original standard devil form, other than a brief transformation, but it could possibly vary to individual on how it works.

Trivia

 * "Majin" is a Japanese word with no direct translation, meaning "one who has a supernatural ability", and it often used to refer to creatures with magical powers such as the Jinn/Djinn (or Genies) in Arabic mythology. The characters in the word individually translate to "demon/demonic, devil, magic/magical" (魔, ma) and "person/people" (人, jin).


 * An 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.