The demons mostly looked extremely grotesque. To them, their looks didn't matter since they respected the strong, but for some reason, one of their hobbies entailed wearing human skin.
It was said that the demons derived joy from skinning humans whole and then wearing their skins.
The bodies of the demons resembled humans in the first place, it was just that they were slightly larger. Once they put on the human skins, all they needed was slight disguise and they would be able to walk among humans undetected. If they swallowed a person's brain, they would inherit all their memories as well.
In olden times when the demons ran rampant, they would often devour humans, put on their skins and mingle among other humans. They would wait for opportunities to attack other people, taking joy in the fear on their faces as they realized that the person was already dead.