Fiend

A Fiend, is a devil which has taken over a person's corpse.

Overview
Fiends are devils which have taken over human corpses 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. Similar to how a devil’s appearance and set of powers may not always represent or go with their “name” (concept) the same can apply for fiends.

Fiends will usually retain their original personalities they had as devils. 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, causing a living fiend host, as with Asa Mitaka's possession by the War Devil. It should be noted that both of these types of fiends are extremely rare being the exception not the rule.

With fiends of the living host variety, both personalities exist separately in one body until the devil decides to take 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 or in immense fear. 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. Once they become reborn as a new devil they will usually have the same exact personality as their current incarnation but if they are the rare type of fiend, then 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.

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 Arabian mythology. The characters in the word individually translate to "demon/demonic, devil/devilish/diabolical, magic/magical" and "person/people".
 * An unnamed fiend is described as Bird-Eating Fiend (をべていた) in the second Character Popularity Poll.
 * There are some other fiends who aren't very well known about that work/worked at Public Safety. There was Fushi's unnamed fiend, Seraphim (セラフィム) the bug-looking fiend, Dominion (ドミニオン) the mammal looking fiend, and Virtue (ヴァーチェ) the headless fiend. Fushi's fiend does not have an known appearance but was said to be "too scary to see" and died previously where as the other fiends mentioned can be seen here.
 * Because Aki is a fiend host but only got possessed by 20% of the Gun Devil, the entire Devil does not need to possess a human corpse in order to make a fiend.
 * Despite being seen as similar, fiends are a contrast to hybrids. Where as hybrids are humans who have been given the enhanced powers of a devil and peak immortal condition as a result of their merging, fiends are stuck with the weakness of a human with their powers being extremely lowered as a result of their possession.
 * In the earlier chapters when Public Safety devil hunters first meet Denji, one of them comments on him perhaps having a “devil possession” before being told that is incorrect. While fiends are a form of possession it is unknown if the specific term “devil possession” only applies to fiends as there have been other instances of devils possessing humans outside of just fiends.
 * Given how living fiends seem to be unable to be turned into dolls yet the dead ones can, it seems that the dead bodies that were once fiends either get rendered as human again, albeit still retaining fiend defects, or are simply now transmutable since the devil in them has left the body.