The Devil is the keeper of Hell, a domain for punished souls.
The Devil is in charge of Sam's assignments from Hell. He owns Sam's soul and has threatened to kill Sam's mother if Sam does not obey him. Despite this however, he has a kinder side towards Sam. He encourages Sam's personal development and unlike Sam's real parents does not coddle him. This has led Sam to succed as a bounty hunter where he has failed at many other things in his life. It is revealed in Acid Queen that The Devil's interest in Sam, is highly unusual, as he repeatedly visits Sam personally. The reason for this has yet to be revealed. Also from some reason he seems to have a hatred or at least a disliking for Sam's father.