Tatsuki Fujimoto's Chainsaw Man is best known for its bizarre characters and overarching plot. The premise involves the world being infested with devils who are born out of human fears. Aside from these entities, hybrids and fiends also play an important role in the story.
As such, a fiend refers to a devil who takes over the corpse of a human being, using it to survive in the world. They are essentially undead creatures who possess some of the supernatural abilities of their respective devils. In Chainsaw Man, fans get to see a variety of interesting fiends, such as Blood Fiend, Shark Fiend, Violence Fiend, etc.
Disclaimer: This article contains spoilers from the Chainsaw Man manga.
The world of Chainsaw Man is populated with a variety of fiends who have supernatural abilities
As mentioned earlier, in Chainsaw Man, a fiend refers to a devil that takes over a human corpse in a desperate bid to exist in the living world. Fiends possess the supernatural abilities of their respective devils but to a lesser degree. After becoming a fiend, devils lose a part of their power and the ability to make contracts.
A fiend can be distinguished from their appearance as they usually have some odd characteristics like Blood Fiend Power and her horns, Shark Fiend Beam and his shark snout. Moreover, fiends can sometimes have names that do not go with their personalities, such as Violence Fiend being a peace lover.
The original personality of a devil is retained when they turn into a fiend. However, a devil can deliberately choose to retain half of the original corpse's brain to blend with human society. This phenomenon is seen in the case of Asa Mitaka, whose dead body gets possessed by War Devil Yoru. She turns into a living fiend host, with her body containing both her and Yoru's personalities.
In the case of a living host fiend, the devil can take over the entire body at any point in time. However, the takeover process can be impeded if the human host is under high stress or fear. When a fiend dies, the devil who possesses the body gets reincarnated in hell and loses all memories of their past life.
On the other hand, some fiends are able to transform into their original devil form temporarily. They can also restore their health by consuming blood. Blood Fiend Power is able to revive as a devil from her pact of blood consumption with the Chainsaw Man. Upon consuming a certain amount of blood, Power's abilities increase drastically, with extra horns growing from her head.
It is possible that a devil might also be able to leave the body of their deceased host at will without having to die and reincarnate. Yoru promised Asa Mitaka to give her body back in exchange for her helping the War Devil accomplish her goals. The promise made by Yoru could just be a persuasion tactic, or there could be a real possibility that devils are capable of leaving their fiend form at will. However, the series is yet to shed light on this matter.