Mandy is a sharp, brooding 8-year-old girl of vast intelligence and a sinister, manipulative nature. Ruthless and intimidating, Mandy's antagonistic additude 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. Mandy's thorough emotionlessness, however, has sometimes been touched in a few scenarios, such as at one point when Billy was thought to be dead and Mandy's reaction appeared quite nervous and partially mournful. Or when Grim was kidnapped, she gave a sad face expression, shown that she cares about Grim and Billy. 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 (a reference to the Dune book series), 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. She also does not appear to have a visible nose, which has been pointed out a few times throughout the series.
Throughout the series, Mandy rarely smiles, and her facial expression remains perpetually disgruntled. Mandy smiling is shown to be the end of the world; in one episode, after entering a local beauty contest against the queen bee of her classroom, it was revealed that all of reality would be humorously, psychedelically distorted, resulting in the series swapping premises with that of the animated series The Powerpuff Girls. Despite this, Mandy is seen to crack an evil sneer at the end of a few episodes, such as when she becomes the only person left on Earth, and after winning the contest which gave her a servant in Grim.
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.