Fiend

A Fiend is a Devil that has taken over a person's corpse.

Overview
Fiends are Devils that have taken over human corpses as 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 fit the theme of 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. It should be noted that this type of Fiend is extremely rare, being the exception not the rule, as well as how a "mingled" Fiend is still just the Devil who is conscious that managed to absorb the traits and/or memories of the human as the actual human remains dead.

There is also a “type” of condition in the series with the protagonist characters of Part 2 that hasn’t been named yet so it is unconfirmed if it is truly a Fiend or not, but for the sake of simplicity and until anything actually gets confirmed the condition will be referred to as a "Fiend and Fiend Host." 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. 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. It is unknown if, for this unique type of Fiend, if the Fiend's distinctive head feature will only become visible when the Devil's personality is in control of the body or if only the living host of the Fiend can see the distinctive head feature and no one else does.

Should a Fiend die, the Devil that possessed the body generally reincarnates once they reappear 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 can also 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 her accomplish her goal, then she would give her body back as a promise. While this could have been just some way of persuading her to follow her orders, it is possible that War Devil (if she really is now the War Fiend that is) has the rare ability of freely leaving 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 from individual 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 is 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 who does not have a known appearance but was said to be "too scary to see" and died previously. There were also some followers of Chainsaw Man such as Seraphim (セラフィム) who looked like a bug, Dominion (ドミニオン) who looked like a mammal, and Virtue (ヴァーチェ) who was headless, but it is unknown if they were Fiends or anthropomorphic Devils. The appearances of the mysterious followers can be seen here.
 * Because Aki is a Fiend host but only got possessed by 20% of the Gun Devil, it isn't necessary for the entire or complete form of a Devil to possess a human corpse in order to make a Fiend.
 * Despite being seen as similar, Fiends are a contrast to Hybrids. Whereas 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 Devils who are stuck with the weakness of a human and have their supernatural powers extremely lowered as a result of their possession. They also contrast how Hybrids can freely change forms (with enough blood) and how Fiends are (usually) stuck in only one form.
 * In the earlier chapters, when the 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 human again, albeit still retaining Fiend defects, or are simply now transmutable since the Devil in them has left the body.