The Adventures of Tom Sawyer
In this case, it's all in the name: Mary is Tom's "saintly" cousin. (Think Saint Mary, Jesus' mother.) She helps Tom memorize his Bible verses, and has memorized enough herself to receive two Bible prizes in Sunday school. Even when Tom's at his most incorrigible, she can usually find something nice to say on his behalf. She's the antidote to Tom's poisonously annoying brother Sid.