"Lou", is a Demon with unearthly powers. He can crawl on walls like a spider, leap over buildings with a single bound, and inslave anyone's soul into unholy torment after the victim looses a bet with him.
Remember the demon cat from the episode, "Dungeon"? well, he used to be one of Lou's assasins in the second underworld (A.K.A, the Void). The Demon cat was sent to hunt Finn down and kill the human, but was unsuccesful. Lou gets annoyed with the cat's tomfoolery in his mission, and wants to do the work himself, so he tracks Finn and Jake down and they easily makes a bet with him.
Finn was placed in a huge, insidious maze, and Jake is placed as a hostage. Finn didn't know which place to go at first, but easily made it through the maze and battled Lou (Who is riding the Demon cat as a steed) and defeated the demon and promised to set all the tortured and abused souls free.
He is much like our modern day devil, only more agressive in nature. Thats because his name was a short letter term for, "Lucifer". .
Lou is voiced by, Brian Doyle Murry, the voice actor for "The Flying Dutchman" from Spongebob Squarepants.