Mary plays her role mostly in Cinderella II: Dreams Come True as a mouse and as Jaq's love interest. Like the other female mice, she wears a maid-like dress and cloth on her head. She sounds just like any of the other female mice and her name nor identity is never specified in the first movie, Cinderella. In the first movie, she is more likely the one in yellow with the turquoise hat, to whom Jaq gives a signal to tug on the string to the other one. She is also clearly the mouse who straightens the thread with her mouth and aims it inside the needle hole and bites on it, since she does it in both movies.
Mary, much like the other female mice, lacks sexual dimorphism, meaning her features are similar to that of a male mouse character and nothing specifies that she is female other than what she wears. She appears without eyelashes and you never see her ears.
In the sequel, Mary's name and actual identity is revealed and she plays a bigger role than the original. We now know that Mary is a mouse with a blue cloth on her head and a pink dress, who is the love interest of Jaq. According to her character in this movie, she is a very concerned and caring mouse who longs to see her Jaq happy. It is unknown or unclear if Jaq and Mary are even a "pair". It may just be that they are friends who have a growing love. In the movie, she shows fear of not only cats but any other human beings. She also sewed an indigo-colored coat for Jaq to show her love for him. She even kisses Jaq near the end of his story.
In the series finale of At Home With Olaf, a short clip of Cinderella featuring Mary, Perla, and Suzy appears during a montage of heartwarming moments from Disney films playing in Olaf's song "I Am with You".