Mary Anne is a young Caucasian girl with freckles, brown hair in pigtails, and brown eyes. She wears a yellow dress with white lace collar, cuffs, and accents. She also wears large white square-frame glasses.
She is proud of her high IQ of 195 and believes herself well-qualified to take over the town.
Skills and abilities
She is a persuasive leader of younger children. In prison she built up combat abilities, including taking up weight lifting, being able throw a barbell at Krampus before it abducts her, and the ability to kill with her braids.
Scooby-Doo! Mystery Incorporated
Mary Anne wanted to run Crystal Cove her way with no adults so she dressed up as Que Horrifico to scare the adults out of town. When she played the music, the children donned their monster disguises and scared away the parents.
When it came to the monsters cornering Mystery Incorporated, the television was turned on to a cartoon distracting the children. When Que Horrifico tried to run, it was caught by surprise by the police car carrying Sheriff Bronson Stone and Mayor Fred Jones, Sr. and fell into one of the nets. Mystery Incorporated unmasked Que Horrifico revealing Mary Anne who explained her plot to drive away the parents and take over Crystal Cove. One of the children stated that her plan wouldn't have worked anyway. Sheriff Stone then grabbed Mary Anne with handcuffs and took her away while ranting to her about costing the town a fortune with merchandise such as Que Horrifico t-shirts, Que Horrifico Pan Flutes, Que Horrifico Dance CDs, and Que Horrifico... when Mary Anne replied, "Ok ok! I see your point."
When Mystery Incorporated determined that the Krampus was attacking bad children, they sought to protect Mary Anne because she would be an inevitable target for the monster. The gang was able to arrange for Mary Anne to receive a reduced prison sentence in exchange for helping catch the Krampus, but Mary Anne refused on the grounds that it was not logical that she would be in danger in prison. Krampus did attack the prison, however, and kidnapped Mary Anne after scaring her and turning her hair white. The gang tracked Krampus and Mary Anne to an abandoned doll factory and caught the monster, saving Mary Anne. Krampus was revealed to be Charlie the Robot, who had gone missing from the Crystal Cove Spook Museum; the gang could not determine who was controlling him.
Later, Mystery Incorporated revealed to their predecessors in a video message that all of the Krampus attacks were staged to trick them into stealing a case of fake Planispheric Disk pieces while leaving their own real ones vulnerable to be stolen. Mary Anne agreed to help in exchange for a reduced sentence; other key participants were Jason Wyatt, Hot Dog Water, and ex-Mayor Jones. 
- She appears to have sinus problems, as she makes thick, phlegmy sounds when she speaks. In the second season, she's developed the habit of spitting.
- In the end credits of The Song of Mystery, she's credited as Mary Anne Geerdon. In the end credits of Wrath of the Krampus, her name is spelled Mary-Anne Gleardon, but on the letter to the parole board, her name is spelled Mary Anne Gleardan.