It is not enough that I succeed, others must fail.
Amanda "Mandy" is the titular deuteragonist of The Grim Adventures of Billy and Mandy series. She is, in fact, shown to be a pretty vile individual who never smiles (in fact when Mandy smiles it has cataclysmic effects): she treats everyone around her with contempt, especially Grim and Billy - though she does seem to see Billy as a friend of sorts.
Mandy appears to be an average little girl, but is often shown to have an almost supernatural power of her own, it is suggested in some stories she may even be the embodiment of evil itself (though this is never proven), Mandy is capable of terrifying people with a mere glance and tends to display little to no emotion outside her usual contempt.
Mandy seeks to someday rule the world and hates being restrained - in fact trying to restrain Mandy is probably one of the most ill-advised things a person can do, as despite her cold and often emotionless exterior she is more than capable of coming up with plans for hurting any person foolish enough to get on her bad side.
In history between Grim Adventures of Billy and Mandy and Underfist, due to the lack of president in the United States, elections are held to elect a new president. Mandy is presented and manages to win. After that, she sends transfer the Dome of the White House to her own house in Endsville, turning this home into the New White House. She also changes the design of the Flag of the United States for a style more to her, with pink and yellow stripes and replacing the 50 stars on a blue silhouette of her face and name to Billy and Grim as her mad scientists, building a laboratory under her home. As is shown in the episode "Mandy Merciles" it is known what would happen after.
In said episode, Billy, Mandy, Grim and Irwin decide to look into the future for fun because they are hot and bored. Soon Billy, Grim and Irwin will regret having done so because in the future Mandy rules the world in the dark form of an immortal hybrid half human half worm demon, leads an army of current Mandy-resembling robots and creates mutants for combat resistance. Grim is still her "best friend" but also her right-hand and she has created a complex line of Billy clones. Mandy tells one of the "Billies" that, in the past, she discovered in their research that combining her DNA with a worm she would be immortal. Mandy's source of power is cinnamon and she has enslaved humanity to extract it for her from mines. She lies to Billy about cinnamon's only weakness saying it is the frogs. Grim leads Billy into the team of resistance against Mandy, and Billy tells them about the frogs, leading them to attack her with them and getting arrested by Mandy. Mandy sentences Grim to the worst of his systems torture for treason and winks devilishly knowing that current Mandy is seeing her.
Although it seems that Mandy accomplishes all her goals and plans in the future, the ending of the episode "Modern Primitives", set millions of years into the future, depict the Earth to be abandoned, with no trace of any human, with only Billy and Fred Flintstone remaining frozen underground until some aliens discover them. This could be, as such, an indication that perhaps sometime in the future, Mandy may have been overthrown and/or died, but this is up for debate.
Mandy President of U.S.A.
Mandy planning universal domination.
Mandy Worm with her right-hand Grim and one of her Billies
Mandy is easily considered, alongside Aku, HIM, Rob and Grandfather, one of the most dangerous Cartoon Network villains given what her goals are, her cruelty and her ability to get away with her crimes most of the time.
Mandy has a couple of similarities to Gaz from the Nickelodeon series Invader Zim due their dark nature and appearance. However, unlike Mandy, Gaz has been seen smiling sometimes and has been confirmed to actually feel love and positive emotions, just having a big ego, whereas Mandy is depicted as only smiling when she gets what she wants, has no respect for her family and never feels remorse for her actions.
In the earlier seasons of The Grim Adventures of Billy & Mandy, Mandy wasn't as evil as she then appeared in later seasons of the show. In those episodes, Mandy was generally depicted as a mean, and sometimes cruel, girl who was occasionally shunned by her friends for being female (like when they didn't let her play with them a CLUE-like game nor baseball due her gender). However, as the show continued its airing, Mandy evolved from a grumpy and vile little girl into a sadistic control freak who desired power at all costs even if that meant enslave the whole planet.
Even so, Mandy is not irredeemably evil like most characters see her, as she has sometimes shown that she cares for others, especially Billy, whom she not only protects but regards as a true friend to the point of cloning him so he can accompany her in the future. Furthermore, Mandy is also capable of feeling love, as shown when she falls in love for Piff.
Though Mandy's smile causes the fabric of reality to tear, this rule is contradicted in the episode "Get Out of My Head", where Billy steals Mandy's body and makes her smile frequently. It can be assumed that dimensional tears only happen when Mandy smiles and is in control of her body.
Coincidentally, Mandy winning the 2008 United States of America presidential elections as seen in Underfist: Halloween Bash were depicted around the same time Barack Obama won the real-life presidential elections that year, which could suggest that in the Grim & Evil universe, Obama didn't win such elections (and by extension never became president, as Mandy's presidency is seemingly later depicted to have developed into a dictatorship as seen in "Mandy the Merciless"). However, in real-life, it would be impossible for a kid like Mandy to win the elections as underage are not allowed to postulate.