Mary is Arnold's older sister who lives in the basement of her parent's house. While Arnold draws cartoons, Mary loves to read and write romance novels – though she hides this from almost everyone.
Inspired by Arnold, Mary decides to leave her home in Wellpinit and run away to Montana to marry a Flathead Indian. Unlike Junior, though, she doesn't completely leave the reservation. She still lives on one when she gets to Montana.
Mary is in the process of writing her life story, though she doesn't get to complete her book. She dies in an accidental trailer fire, an event that completely devastates Arnold (Chapter 27).