Who is Mavis McCall in the book Summer of My German Soldier?

In the book Summer of My German Soldier, the main character Patty is sent to The Jasper E. Conrad Arkansas Reformatory for Girls. Mavis McCall is Patty's roommate there. Patty describes Mavis as having "Raggedy Ann eyes" (Chapter 20).
Mavis McCall sometimes gets on Patty's nerves in the room they share together. Mavis calls Patty "Natz," which she does not like. She expresses this to Mavis, but her roommate notes that at least she does not call Patty "Nazi" or "spy" like the others at the school do. Mavis has a large appetite, which Patty discovers at mealtime. At breakfast, Mavis eats every crumb of her food. After Mavis finishes her slice of bread, Patty offers to share her own plate of food. Mavis eats Patty's grits and eggs. By contrast, Patty eats very little food.