WARNING: The following contains spoilers for Chainsaw Man #84 by Tatsuki Fujimoto, Amanda Haley and Sabrina Heep, available now from Viz Media.
Humans and Devils populate the world of Chainsaw Man, but they can combine to form two different creatures: Fiends and Devil Hybrids. Due to their similarities and the rarity of Hybrids, the distinction between the two is often vague. While Fiends and Devil Hybrids share some commonalities, there are key differences that help distinguish them from one another, and even from the Devils who formed them.
Devils are supernatural creatures who possess unique abilities, like the Future Devil, who has visions of the future. Humans can enter into contracts with them to call upon their powers, but a Devil's strength is determined by how much fear their name garners. A prime example is the Gun Devil, who grew so powerful that every nation enacted strict gun laws and restricted news coverage of violent crimes to curtail humans' rising fears. Devils are virtually immortal since they live in an endless cycle of dying in the human world and returning to Hell, where they can die again only to return to the human world.
If a Devil possesses the body of a human corpse, they become a Fiend. While the Devil's personality usually stays in control, some Fiends retain more of their host's brain, as Aki did after becoming a Gun Fiend. Fiends are significantly stronger than humans and can use their Devil's abilities, but aren't nearly as powerful as their original forms, so it's often considered a last resort for survival. As Fiends, they aren't able to make contracts with humans or Devils.
Of all the Fiends introduced in the series, it's Power who makes it to the top of most fan's lists. Her chaotic charm aside, Power is a Blood Fiend who can freely manipulate the blood from her body to form various weapons. She, like all Fiends, has a distinctive head feature, which in her case are the two horns that sprout from the top of her head and vary in size depending on the amount of blood she's consumed. For Beam, the Chainsaw Devil-worshipping Shark Fiend, he has a shark-shaped head, can swim through solid objects and has the uncommon ability to temporarily change into his original Devil form.
Devil Hybrids, on the other hand, are relatively rare, with only four of them appearing within Chainsaw Man so far. They occur when a Devil fuses with a human. Unlike their Fiend counterparts, Hybrids retain their human consciousness and appearance but add much of the Devil's original strength and ability to transform. They are also able to restore their strength and regenerate by consuming blood, which both Devils and Fiends can do as well. However, when they are low on blood, they can't fully take on their Devil forms.
Denji is the standout example of a Devil Hybrid and informs most of our understanding of the rare fusion. Before combining with Pochita, the dog-like Chainsaw Devil, Denji was missing an eye and an assortment of organs and left for dead. The contract between the two fused them into one, healing Denji and granting him the power to sprout chainsaws from his head, arms and legs. While initially capable of a partial transformation into a hybrid form, in recent chapters, Denji's gained the ability to transform into a more complete and menacing Chainsaw Devil, losing much of his human appearance.
All the Devil Hybrids have unique triggers to begin their transformations, with Denji's being a starter cord in his chest that he pulls to rev up his inner chainsaw. Reze, the Bomb Devil Hybrid, pulls a grenade-like pin in her neck, and Katana Man unsheathes his bladed left hand. The fourth Hybrid is Quanxi, a Chinese Devil Hunter, whose Devil counterpart is never named but requires pulling a strange arrow out of her empty eye socket, morphing her into a sharp mess of blades and arrows.
In a sense, Devil Hybrids could be seen as a complete version of Fiends since the most significant difference lies in their strength. While becoming a Fiend is often seen as a last resort, when you have Cosmo the Cosmos Fiend able to impose the understanding of everything in the universe on her target, it becomes evident that both forms are a serious force to be reckoned with.