Mandy is a dark, heartless young girl of vast intelligence and a sinister, manipulative nature. Demanding and rude, Mandy's antagonistic behavior has left her parents in trembling, quavering perpetual intimidation, and her frightening, cold nature has left her virtually fearless in times of severe supernatural danger or while being faced with menacing, bloodthirsty otherworldly monsters. She also possesses a strong lust for power as revealed in an episode providing insight to the future of the town of Endsville, in which Mandy has enslaved all of its citizens and evolved herself into a large, anthropomorphic being resembling a giant caterpillar, and Grim's abilities have helped her gain access to satisfaction of her desires. Mandy has been known to domineer everybody around her without fear, and the few things that she likes include junk food, television, and all things dark and melancholy. Throughout the series, Mandy NEVER smiles, and her facial expression remains perpetually disgruntled. Mandy also despises Billy's best friend Irwin, who is strongly infatuated with her, and it was revealed in a television special that her worst nightmare would be to grow into a plump, optimistic woman married to a brawny adult version of Irwin, and in another episode it was implied that Mandy harbors a mild phobia of figure skating.

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