The Devil (sometimes going by the human alias Jerry Belvedere and sometime by his previous angelic name Lucifer) 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, let alone, being constantly 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. Even though he's not around, most people (and demons) seem pretty terrified of him.
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 nicy-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, claiming the only true love is the angels loving God (which he apparently still does or he just really, really misses ice cream as God cursed him so it vanished whenever he touched it), and he has at least once shown disdain for the mortal sins he otherwise tends to encourage wholeheartedly.
The Devil is immortal, can see or hear anything he wants (outside a circle) and is pretty much impossible to kill or damage (he doesn't get a bruise when Sam takes several swings at him with a baseball bat.) He can teleport himself anywhere and uses telekinesis to make all the shelves in the Work Bench fall of top of Sam.
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) In a new hope after Sam captures 40 souls in Hell he's in a good mood and wanted to go for a drink the Devil stopped the car and and Sam was told to get out and rain started to fall on oders of the Devil, but the more he lets Sam get away with, the less likely that explanation seems (and Sam gets away with a LOT, even defying him openly.) 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 or hear inside of 'em) and the sword of the Archangel Michael, which he has tucked away somewhere so good luck trying to get him with that one. (See "Rebellion".) In Season 2, nobody has used the circle ever again, as if the writing staff decided to abandon the idea. His biggest weakness as shown in the last episodes of Season 2, is his own vanity. The Devil cannot look at himself in the mirror as his vanity distracts him from all else.
Also strongly distrusts bears, for some unknown reason. Then again, do you trust bears? The Devil has a bunch of children, but only two are known, Sam and Morgan. (Actually, Sam is not really the Devil's son however)
Being the Devil, he can do almost anything. The only limits are is that he can't forcibly drag people down to hell (they must sin and then die or sign the Devil's contract), he can't hear or see inside a circle.
In one instance, Nina has referred to him as the second most powerful entity in the universe.
He is well nigh immortal and indestructable (he was able to get hit by Sam with a baseball bat without any damage or reaction). But it is said that the Sword of Michael is one of few things that can kill him.
He can kill demons easily by turning them to dust.