Mary, Mary, Quite Contrary
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:
"Oh, she loved to write short stories. Little romantic stories. She wouldn't let anybody read them. But she'd always be scribbling in her notebook."
"Wow," I said. That was all I could say. I mean, my sister had become a humanoid underground dweller. There wasn't much romance in that. Or maybe there was. Maybe my sister read romances all day. Maybe she was trapped in those romances. (5.92-5.95)
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.