The Devil (sometimes going by the human alias Jerry Belvedere) is the keeper of Hell, the afterlife destination for all damned human souls... provided his minions can actually keep said souls in check.
Sometimes (okay, a lot of the time) they can’t, and that irritates him. His relationship with his employees is pretty lousy: they don’t seem to see much of him (it’s a shock to Steve that Sam had ever even spoken to the Devil, forget constantly being bothered by him), he resents when they take time off from work (to party on Halloween), and apparently it’s damned tough for the Devil to get a decent cup of coffee. Also that one time a whole bunch of them tried to overthrow him with the Sword of Michael. That was rough.
Ironically he seems to get along better with humans, although possibly only because humans are easier to manipulate. He maintained an on-again-off-again relationship with a woman named Mimi Hansen for about twenty years before she finally broke it off with him and moved to New Mexico. (It is still vaguely possible that her daughter Cady was the result of their flighty union.) He’s also charmed a failure of a rock musician with the intention of buying his soul, and played nicey-nice with an almost equally charming (but equally dangerous) escapee from Hell.
He claims to know how it all ends—“God wins"— and that he and God have reached an agreement: The Devil can play with God’s “toys,” (humans), as long as he stays out of God’s “house,” (churches or other places of worship, presumably). He may have slightly conflicted feelings about his relationship with God (or he just really, really misses ice cream), and he has at least once shown disdain for the mortal sins he otherwise tends to encourage wholeheartedly.
The Devil likes rock music, Bloomin’ Onion and harassing Sam Oliver. At first this seems to be the basis for his entire relationship with Sam (cheap laughs), but the more he lets Sam get away with, the less likely that explanation seems. He’s a sucker for Sam’s puppy dog eyes and when Sam finally asks for the truth about their potential biological relationship, The Devil is as straightforward as he’s ever been: he can’t promise he won’t lie, because he’s The Devil.
His only other weaknesses are circles (he can’t see inside of them) and the sword of the Archangel Michael. The real one, not a fake from Koreatown. That one won’t work.
Also, possibly bears.