In The Bridge to Terabithia, May Belle Aarons is the younger sister of the book's protagonist, Jesse Oliver Aarons. We learn in the first chapter that May Belle worships Jesse, which doesn't bother Jesse too much since his older two sisters despise him and his youngest sister is practically a baby and cries "if you looked at her cross-eyed."
May Belle can sometimes be a nuisance since she is so frequently intervening in Jesse's life out of the desire to be around the brother she admires so much. She constantly tries to tag along with Jesse and Leslie, and if she is left out of their friendship and games, she turns to sulking. As Leslie and Jesse grow closer, May Belle mopes more frequently, and Jesse starts to feel that her wish to always be included is oppressive. Despite this, Jesse and Leslie both serve as protective forces for the girl.