Although Leslie's primary role is as guide/mentor, she also serves as a foil in contrast to May Belle. Both girls want to be friends with Jess, and both are alone in their age group in seeing his value and special qualities. He loves both of them, in turn, and they're the people he most cares about being generous with at Christmas, giving one a puppy and one a Barbie. But Jess pushes May Belle aside in favor of Leslie, leaving his little sister friendless in his quest to reach out and have a friend for himself. May Belle observes and tries to participate, but he and Leslie placate her with paper dolls, or he threatens her with punishments.
Yet, when Leslie's gone, it's May Belle who reaches out to Jess in his grief and tries to follow him to Terabithia. By helping May Belle and acting more like a real friend to her, Jess gets to hold on to his friendship with Leslie and honor her memory by treating May Belle the way that Leslie treated him, continuing to share the valuable knowledge that Leslie exposed him to.