Mary is the Monsters next door neighbor.
Mary has shoulder length black hair and wears t-shirts and shorts. She is voiced by Tara Strong, who voiced Raven from Teen Titans, Ben 10 from the first Ben 10 series, and Timmy Turner from The Fairly OddParents. In the upcoming CGI animated film, she will be voiced by Dove Cameron, who is known for Liv and Maddie and Descendants.
Mary is a friendly person, she says things like "Groovy!", "Far out!" and "Out of sight!" a lot. These were phrases used in the 1960's which may indicate that someone close to her like a family member may be a former hippy. She likes to jump on her trampoline in her backyard. Mary is often shown to be very wise for her age as she often gives the monsters good advice such as not letting their fears get the best of them.