In Buffy The Vampire Slayer (Season 7), a preacher named Caleb turns up who is super strong and hard to kill. Was he a demon or something, or just a really powerful human?
Ok, major spoilers:
Caleb was an ex-priest and serial killer who would periodically merge with a non corporeal entity called the First Evil.
This in turn temporarily gave him all of his super powers.
Caleb described the First Evil as being sin itself.
So to answer your question, Caleb was a very charismatic human serial killer who merged with a powerful evil entity.
Caleb was human.
His entry in the Buffy the Vampire Slayer Encyclopedia: The Ultimate Guide to the Buffyverse describes him thusly
Defrocked priest who began his evil career as a serial killer of women attracted to his charm and religious message
Periodically merging with the First, Caleb possessed superhuman strength.
In the absence of anything to describe him other than human, we can assume that he's human.