Mandy

Mandy is the titular tritagonist of The Grim Adventures of Billy and Mandy, who is best friends with Billy and Grim. She is a devious girl from Endsville. In stark contrast to her cheerful, talkative, uniquely stupid best friend Billy, she is shown to be intelligent, quiet, bitter and utterly ruthless with no redeeming qualities whatsoever.

After the theme song, Mandy is shown in front of a black screen, usually speaking to the viewers in a creepy, sarcastic way, similar to the couch gag opening of The Simpsons.

Personality
Mandy is shown to be an anti-hero at best, and an antagonist at worst. An irredeemably cruel, intelligent, cold-hearted, grouchy, abusive, bitter, callous, cynical, heartless, merciless, vain, rude, spoiled, self-centered, stubborn, mean, arrogant, unforgiving, unsympathetic and slothful little girl, she regards herself as being superior to many of the characters on the show, even her parents. She aspires to dominate the planet with an iron fist and to stay as far away from Irwin as possible. Mandy’s dreadful behavior has left her parents in fear and they are often shown avoiding her. Her mother spoils her rotten and showers her with money, compliments, and expensive gifts, despite her fears, and her father is stuck cooking and cleaning for her. However, it is shown that Mandy truly loves her family, as she was genuinely shocked when they revealed they were afraid of her. As much of a psychopath as she is, Mandy doesn’t like cheating and would rather accomplish things for herself. It is said by many fans that Mandy gets scarier and meaner every season.

Mandy also has superior intelligence and strength not to be reckoned with. She has an almost superhuman force as she is able to battle monsters with ease as well as being able to understand complex theories of the world when referring to her superior intelligence, despite being a child, so she might be a prodigy. She also never misses a chance to exploit the stupid, which more or less justifies her attachment to Billy.

While Billy gets himself/the trio into trouble, she is usually the one to get him/them out of it, along with Grim who, without her force, would not even bother trying to help. According to Billy and Grim, Mandy apparently has a fear of professional figure skaters--but she denies this, claiming that she simply "doesn't trust the way they spin," although, in Billy & Mandy's Big Boogey Adventure, her true worst fear is depicted as growing-up into a kind, chubby, unattractive old woman and settling-down with Irwin. It is also shown in the movie that Grim and Billy are actually very meaningful to her and she feels truly alone and lost without them.

Mandy was the main antagonist of the crossover episode The Grim Adventures of the Kids Next Door, as she took over the Kids Next Door’s Moonbase and nearly dominated the entire Cartoon Network multiverse. This is probably Mandy's most villainous role ever.

Although Mandy rarely smiles, she is shown to smirk in times when she wins or if she is concocting a very sinister plan. When she did try to force a smile in a beauty pageant, it caused a rip in the dimension that forced her, Grim, Billy and Irwin to transition into the Powerpuff Girls world, due to the sheer awe of it being extraordinary.