Mandy is the titular tritagonist of the franchise, 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.
When Mandy was a baby, she was shown to have never smiled; she was even visibly unhappy in her mother’s womb. When she does smile, it throws off the natural order of the universe as seen in one episode, My Fair Mandy. However, this did not apply in some earlier episodes, where she smiled only to her friends Billy and Grim. However, she had to smile in front of everybody in the school auditorium when she sang Over the Rainbow as part of a pageant.
In the episode The Problem with Billy, it was shown that she first met her best friend Billy at an early age, when Billy had repeatedly rung her doorbell until she was annoyed enough to answer. Later, Billy stole her feeding bottle, put it in his mouth and began flushing it like a toilet. When she did, he immediately pointed out that she had no nose and shoved an ice cream cone into her face after she had asked him if he was "some kinda idiot?". She punched him, called him a creep, and slammed the door in his face. He simply giggled in response to this, telling "she digs me", thinking that she liked him.
Years later, she and Billy had met Grim on the day of the tenth birthday of Billy’s pet hamster, Mr. Snuggles. Grim had appeared to reap the soul of the hamster as it was time for the hamster “to kick the bucket” as Mandy had said earlier in the episode, but neither the hamster nor the duo would give in. So, they made a bet. If Grim beat them in a game of limbo, he could have Mr. Snuggles and Billy, but if the kids won Grim had to be their best friend forever. However, the kids won after Mandy cheated in the limbo contest and Grim has been stuck with them since. In the episode Sickly Sweet, Mandy turns into a monster with the combination of Hippopotamus, Alligator, Bear, and Iguana by "The Mask of Beast" for her cynical attitude with Billy and Grim.
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.
Even though Mandy is controlling, manipulative, wrathful, even devilish at times, she seems like your average little girl wearing a pink dress with a yellow flower in the center, black Mary Jane shoes, white socks, and blonde hair accessorized with a black headband. Although she has black eyebrows that are thick and they are almost always furrowed and a frown to match which gives her a scowling expression most of the time, and her hair is curved into a style resembling devil horns.
In the show's regular art style, she has no visible nose, but small nostrils. She has black pupils, but on some occasions, she is shown to have blue eyes. Such as in My Peeps, after zapping Billy with his scythe, she and Grim are both seen in anime form revealing Mandy to have blue eyes and a small angular nose. In Underfist, Mandy's eyebrows are thinner, her eyes are bigger and her head is wider.
- Despite her lack of supernatural powers, Mandy primarily relies on using her intelligence and intimidating demeanor to get her way and resolve difficult challenges. She is capable of using her wits and boldness to exploit the mental and emotional weaknesses of certain individuals and then uses her intimidating appearance and personality to weaken their resolve and thus allow her to gain the upper hand over them or even seemingly control their minds due to the emotional scars she may cause them, however this usually does not work on individuals of equal intelligence to her or those who have a strong will.
- Being that she is still human, there are still several occasions in which Mandy is outwitted, imprisoned or about to be destroyed, at which point she relies on either her wits to think of a way out or on Grim and Billy to help her out or by manipulating them or someone else nearby. Considering that most of the other characters in the series tend to lack common sense, Mandy is able to figure out simple solutions that others were usually oblivious to.
- Mandy also has superior physical strength and martial arts skills which she can use to best most ordinary humans and even some large monsters, but her most notable ability is her intimidation and manipulation skills.
- It appears that Mandy's behavior has some sort of relation to the natural order of the universe, as shown in the episode, "My Fair Mandy" where it is shown that if she smiles against her own will, the very fabric of the universe will unravel and warp itself into a completely new reality.
- Like a few characters in the series, Mandy usually gains temporary powers via supernatural, technological or magical means, such as via Grim's Scythe, ancient artifacts or some sort of vehicle, weapon or robot. Her most notable power boosts were those she gained through artifacts such as the Chicken Ball and the Left Hand of Horror (in an alternate future in which she became The Lord of Horror), or when she fused with the Brain Eating Meteor and the Delightful Reaper. In an alternate future, she even found a way to make herself immortal by combining her body with worm DNA, after which she proceeded to conquer the planet.
- When she tries to force a smile, it has the power to rip the dimension she is in demonstrated in the beauty pageant in the episode "My Fair Mandy" making a vortex to transition the cartoon into the Powerpuff Girls' world. This is due to the disruption of balance in the world that she does not smile ever. However, in the first season, Mandy was seen smiling.
Billy has been Mandy's best friend since they were infants. Sadly, she considers Billy her idiotic henchman to do her every bidding as she never misses the chance to benefit from his astonishingly low IQ of -5. He usually complies and does everything she demands of him, seeming to either not mind and at times possibly even enjoy it. He rarely ever snaps at her for this, once, however, he did shout at her as a result of her treatment of him was when she had told him "Thank you". The other time was when he grew tired of her controlling behavior when she and her parents are forced to stay at Billy's house while their own home is being fumigated. Billy may be responsible for Irwin's crush on Mandy, since he flirted with her (in Mandy's body) in the episode Get Out of My Head!
Mandy tends to be that of a tsundere towards Billy. Despite her negative, cruel behavior towards him, Mandy has been shown to care for Billy, and the feeling is mutual. Her feelings could be completely platonic since she mostly treats him nothing more than a servant. But Billy is the only person who Mandy allows to have physical contact with her, embracing her on numerous occasions, even kissing her on the lips a few times. Mandy has also been known to read Billy bedtime stories in the middle of the night when he couldn't sleep whether it be from eating nineteen boxes of cupcakes or other reasons. He has also suggested wanting to marry her at least three times. She also always helps Billy when he gets into trouble, that is until she becomes too bothered to continue and just stops or leaves the scene.
In Billy & Mandy's Big Boogey Adventure, when Billy is supposedly eaten by a giant sea creature, she is in denial of his death saying, "Billy's always fine." But when he's been missing for six hours and Irwin's becoming restless; she starts to get worried herself as she gazes into the water and tears up, believing him to actually be dead. Moments later, Billy appears out of the water riding the Kraken, laughing and screaming "I'm fine!" Mandy gasps, pleased and surprised to see him again, but quickly denies ever being worried in the first place since of course, Billy is always fine. Near the end the main characters must face and conquer their worst fears. Mandy's worst fears are revealed to be her becoming fat, unattractive, jolly, and happily married to Irwin. It was in this movie that she realized that she can love another person besides herself, and it isn't necessarily a bad thing, and she faces her feelings toward either Billy or Irwin.
Grim initially loathed both Billy and Mandy. Mandy does not seem to care about his hatred and simply uses him for her own purposes. Despite this, the two can come to an understanding on certain things, such as exploiting Billy's stupidity. Grim, like Billy, is more of a servant to Mandy. Unlike Billy, however, Grim regularly stands up to Mandy. The reaper is shown to regularly fantasize about the duo's ultimate deaths, but overtime, he grew to genuinely care about them, an example being in Fiend is Like Friend Without the "R" when Nergal kidnapped Billy and Mandy for himself and Grim came to their rescue.
Mandy's sworn enemy, Mindy is an obnoxious, self-absorbed, stereotypical popular girl. She and Mandy always seem to be rivaling each other in almost every episode Mindy is actually in. It is revealed that Mindy once wanted to be Mandy's friend, but sore from rejection, she has grown to hate Mandy and sought revenge. Ever since then, the two have been arch rivals. Despite being just as mean and spoiled as Mandy, Mindy is actually a cowardly airhead, as opposed to Mandy, who is a fearless genius. In Wrath of the Spider Queen, Mindy is shown to be able to form a temporary alliance with Mandy if it means survival, when she fights spiders alongside her nemesis.
Irwin has tried to make Mandy his girlfriend on a few occasions, but she always refuses in disgust and usually ends up beating him up. Repulsed by his pursuits to win her affections, Mandy chooses to be as far away from Irwin as possible. He makes it somewhat difficult as he appears in every other episode and is close friends with Billy. Irwin had first fallen for Mandy while she was possessed by Billy in Get Out of My Head! when Billy (while possessing Mandy) had called him a "cutie" and flirted with him.
In Scythe For Sale, Irwin uses Grim's scythe to cast a spell on Mandy that made her fall in love with him. She almost becomes his wife in a forced wedding ceremony, but Billy manages to break the spell over Mandy, restoring her to normal.
In the first movie, Mandy's worst nightmare is shown to be becoming fat, unattractive, kind, and married to Irwin. She fears that she'll someday be in love, and she actually faces and overcomes this fear, but it is unclear whether she's accepting her feelings for Irwin or Billy.
In the final episode, Mandy actually starts to fall for Irwin when she learns that he was born evil (possibly due to his monster heritage), but quickly goes back to normal after she kisses him.
Saliva is Mandy's dog. Mandy cares deeply for her dog and loves him very much. Saliva is the only living creature that Mandy shows any love, emotion or affection to. She often makes Grim play with him, like letting Saliva bury his bones in the yard.
Mandy seems to like Jeff and always helps him with his problems regarding his father. Unlike Billy, Mandy is a lot kinder to Jeff than Billy is, and does not jab Jeff with a stick like Billy does.
- Mandy is the only character to officially not have a designated nose, (although Jack does not have a nose after his original head is cut off) though nostrils are present when in close-ups.
- Other than being the deuteragonist most of the series, Mandy is also the antagonist in some episodes.
- During the first few seasons of the show, some fans mistakenly thought that Billy and Mandy were brother and sister. This was made fun of in a later episode when Harold told Gladys that he thought Mandy was their daughter.
- If Billy and Mandy were siblings, Mandy's full name would be "Amanda Uglio".
- The show's description on Netflix has accidentally referred to Billy and Mandy as siblings a few times.
- According to her father, a pack of wolves tried to raise her as one of their own when she was born.
- Her voice used to be more scratchy and high in Grim & Evil, but then her voice became more deep and melancholy.
- Mandy likes playing a Hoss Delgado video game, President Evil which is a parody of Capcom's Resident Evil (1996).
- Mandy is often interpreted as a "Goth" by fans, although it has never been confirmed and the connection is based purely on the stereotype of Goths being overtly cynical, merciless, ill-tempered and angry. However, she does read Goth magazines.
- Although it has been stated the Mandy never smiles, earlier episodes, comic stories, and some Cartoon Network bumpers depict her grinning mischievously after getting her way.
- This has lead people to speculate why Mandy managed to throw the universe off balance in "My Fair Mandy" when she smiled; many presume that this was because this wasn't a natural smile.
- Mandy has never been seen laughing except in the end of Test of Time where Billy became involved in human evolution
- Mandy might be wearing glasses when she becomes an adult, this was shown in two occasions.
- Mandy has a couple of similarities to Gaz from the Nickelodeon series Invader Zim.
- Unlike Gaz, she‘s much more heroic than her.
- Mandy may be younger than Billy since his birthday party was celebrated in the episode "Tastes Like Chicken"; and her party was celebrated in episode 9, "A Grim Surprise", even though her actual birthdate was not for another "five months."
- Mandy appeared in Foster's Home for Imaginary Friends episode, "The Big Picture".
- Mandy is mentioned by Uncle Grandpa in the Steven Universe episode "Say Uncle".
- In the episode Anger Mismanagement, it is revealed that Mandy as well as Grim have gone to anger management classes.