Too Dumb To Gamble: Maybe if he never wagered his own freedom, he would still be reaping souls.
Of course, he hadn't counted on Billy or Mandy cheating to win by getting Billy's ancient hamster to attack Grim during the game.
"Still"? Plenty of episodes explicitly show that he still does his job of reaping souls, albeit with mixed success because Billy and Mandy tend to interrupt him when he's doing so and let his victims get away.
Unexplained Accent: Grim's Jamaican accent. His grandmother also has one, and has a Jamaican look to her to boot.
Chaotic Stupid: When Billy takes Mandy's Nerve, he gleefully abuses Grim, implying that a lack of courage was the only thing that stopped him from doing so before. He also has an "inner frat boy" that advises him to beat the crap out of anything he doesn't understand.
Arbitrary Skepticism: Despite living in a world where supernatural elements are commonplace, and being friends with the Grim Reaper himself, Mandy adamantly refuses to believe in such things as Santa Claus and the Tooth Fairy.
Likewise she does this to the "Delightful Reaper" in the KND crossover when she realizes it was too powerful for Kids Next Door's weaponry and willingly let it assimilate her. Like it the above example, it takes on her appearance.
Improbable Weapon User: In the Cartoon Network game "The Fight Before Christmas", she uses a toy hammer, a toy plane, a dollnote which she hurls like a throwing star, a fistful of marbles and a pan of popcorn as weapons against the evil toys overrunning Santa's workshop.
Iron Lady: Her ultimate goal in life is "[....]to do somethingimportant!"She eventually is elected president of the United States, and in the distant future, will rule the world.
Jerkass: More and more after each season. Beyond a shadow of a doubt. Her beast transformation in "Sickly Sweet" even makes her sound like a literal ass (the animal). Hell, that episode shows that even trying to do anything nice is nearly impossible for Mandy, to the point where she has physical difficulty just trying to say "Please."
Jerk with a Heart of Jerk: While she does do heroic actions like saving Billy from himself or whatever Monster of the Week they're up against, as well as saving the whole planet, Mandy will always have a selfish motive in mind no matter what.
Madden Into Misanthropy: It's implied that three things helped complete Mandy's metamorphosis into the cynical tyrant we all know and love: The realization that her beloved unicorn toys were being churned out daily by a factory, Mindy's inane, meaningless prattle, and having to deal with Billy every single day of her life.
Hunk: The movie suggests that he will grow up to be this. When Mandy faces a vision of her worst fear — growing up to be a cheerful, friendly adult — she is horrified to learn that her future self is married to an adult Irwin. And that he's gorgeous.
The goddess of chaos. She possesses the powerful Apple of Discord, a golden apple which can transform into any shape to perpetuate chaos. Grim used to have a crush on her during earlier episodes, but later changes his mind, claiming her to be a "psychopath". She is also in love with Hoss Delgado.
An old arch-enemy of Grim's since junior high school, and a former school bully, Boogey played pranks on Grim throughout his school career. Boogey is, in fact, the Boogeyman himself, and can shapeshift in order to scare people.
Big Bad: Of the first movie (interviews with Maxwell stated that Wrath of Khan was what inspired him to use him since he appeared in only one episode). He was also indirectly responsible for the events in the second one.
A black-skinned supernatural being with green eyes and wears a business suit. With the ability to shapeshift he can create several tentacles from his back that can electrocute his victims or turn them into beings looking like himself. What Nergal desires most is having friends, once stating that it is quite lonely in the center of the world.
Hoist by His Own Petard: When Grim came to collect him the first time, he managed to stop him with a feather leading into the deal to give him immortal life. At the end of the special, Grim uses a feather to send Jack to the underworld.