Mary is a human character featured in the The Muppets.
She works as a shop teacher at an elementary school, and in one scene, shows her students how to fix a 12-volt starter on an old station wagon. As a testament to her teaching skills, her students are disappointed when the school lets out for spring break, but she cheers them up when she tells them they'll be back in two weeks. According to the production notes, her sixteen students account for the sixteen perfect apples that sit on the edge of her desk.
In the movie, it's revealed that Mary is concerned of Gary's relationship with Walter and feels that he doesn't spend enough time with her as much as he does with his brother. When Mary goes home after being neglected one too many times, Gary learns a lesson about not taking the person you love for granted and rekindles his relationship with Mary, which motivates Kermit into admitting that he needs Miss Piggy. At the end, Gary proposes to Mary, just like Mary had dreamed about earlier in the film.
In The Muppets: The Movie Junior Novel, her being a shop teacher is one of the reasons Gary states that he and Mary are perfect to help rebuild The Muppet Theatre. This fact is omitted in the film. At the end of the movie, Mary uses her abilities to rewire the fuse box to restore electricity to the studio during The Muppet Telethon.
Mary (played by an uncredited stand-in) also appears with Gary at the beginning of Muppets Most Wanted, but they are only seen from behind to reference the fact that they won't be returning.