Latest appearance: The Mysterious Seven Project
Mandy is one of the titular characters in the 2000's television series known as The Grim Adventures of Billy and Mandy. She is one of the current owners of the Grim Reaper, alongside her friend(?) Billy.
In "Meet the Reaper", the first episode of the show, Grim took Billy's hamster for slaughtering, but he and Mandy a deal: if Grim could win against her and Billy in a game (which was decided to be limbo), he could keep the hamster; but if Mandy won, the hamster would be returned and Grim would be their best friend forever. Just when Grim was about to win, however, Mandy told the hamster to "kiss, kiss", and it leaped onto Grim, causing him to panic and hit the limbo stick. Thus, Grim was forced to be Billy and Mandy's best friend for eternity, against his will.
Mandy will do anything to get what she wants, even threaten others. In fact, she seeks to dominate the world one day. But when it is offered to her, she denies, most likely because she wants to do it herself and/or at the right time. Mandy has an unusually large knowledge of real-world issues, for a young girl.
In this series, Mandy is the secondary antagonist, being a supporter of Mephiles the Dark and his destructive intentions. In Endgame, it is revealed that they made a deal. Mephiles told Mandy that the citizens of the meta-universe would betray her in the future, including Grim. Thus, Mandy agreed to help him destroy Mario and his friends once and for all.
Mandy appears alongside Billy and Grim in Fighters of Lapis 4.
Mysterious X Project
Mandy appears in The Mysterious Five Project with Billy and Grim, but as a minor character.
She later reappears in The Mysterious Seven Project, but by herself this time. She appears in Binary Time as a member of the Uncanny Five, alongside Jack Skellington, Death the Kid, Wolverine, and Blinky.