Beware the Nice Ones: He's pretty nice when not reaping, though it has been made clear on several occasions that despite his attitude and butt monkey status to Billy and Mandy, he is not one to be trifled with when his patience is at an end. Just ask Jack O' Lantern.
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 (unless the nerve also influenced since it was Mandy's). 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.
Creepy Child: She tricked The Grim Reaper into eternal servitude, using him as her maid and his powers for personal gain, and faces down everything from the Boogeyman to Cthulhu with nothing more than a wry comment. She also never smiles, ever, because doing so would cause the universe to implode. She was also able to take over a large evil brain/alien when it ate her brain.
Flanderization: From having no emotions, along with a Hair-Trigger Temper, to a totally sadistic psychopath trying to take over the world, who not only can pretty much fix or undo anything by just appearing on the scene, but owns everyone apparently.
Flat Earth Atheist: She is rendered immune to the Tooth Fairy's powers by revealing, to his face, that she doesn't believe in him.
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.
Informed Deformity: In My Fair Mandy, a skilled make-over artist was shocked by her appearance, at first mistaking her for an ugly tree stump.
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: Not in the first couple of seasons but more and more after each one of them. 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 — but...
Jerk with a Heart of Gold: ...she is also shown to care about Grim and Billy, even though she won't admit it and never talks about it. She also tends to stand up to bullies.
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.
Mama Bear: Mostly in the earlier seasons. If she considers you a danger to Billy, she will do everything in her power to make sure you stay as far away from him as possible. It's one of the main reasons why she hates Grim so much, and wants him out of her house.
Gladys:(after beating Grim over the head with a mop) STAY AWAY FROM MY BABY!!! He's allergic to the undead!
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.
Disproportionate Retribution: When Mindy first moved in Endsville, Mandy actually tried being nice to her because she looked alone. She quickly regretted it hearing her chatterbox antics and told her to shut up. Mindy didn't take it well, and had became her obnoxious rival ever since.
Real Men Wear Pink: He loved to sing showtunes. In "Tricycle of Terror," he expresses a desire to be on Broadway.
Voiced by: David Warner, later Martin Jarvis
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. However, he gets better once he starts a family.
Affably Evil: He's generally polite, even around his victims.
Character Development: Starts off as an Anti-Villain, but ends up becoming a friendly member of Billy's extended family who usually means no harm. This primarily came after he began a family of his own.
Crazy Jealous Guy: "Son, it's not about you liking a girl, or a girl liking you. What's important is that she can never get away from you."
Genre Savvy: He actually manages to see through what he thinks is Mandy's initial attempts to prank him. However he doesn't count on Erwin's clumsiness.
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.
Breakout Character: Introduced as part of the old horror icons alongside Frankenstein's bride and the Wolfman. But thanks to Phil hitting the role so enthusiastically, he made more appearances, completely eclipsing the Bride and Wolfman, even having a family connection to Irwin. He was even used for part of Cartoon Network's live action Halloween promos via a puppet.