May Belle is the only one of Jess's siblings that he has real affection for or tolerates. In turn, she "worship[s him]" (1.8). She's as lonely as Jess is, and also requires a friend: "I need somebody to play with," she tells Jess early on (2.4). But while she often looks to Jess to provide that friendship, for most of the book he's just not that interested, seeing her as a little squirt and preferring to go to Terabithia with Leslie. (For more on how she affects his friendship dynamic with Leslie, see the section "Character Roles.")
After Leslie dies, May Belle is still there for Jess. No one listens to her when she says Jess wasn't in danger with Leslie, and she's the only one who worries about him being lonely, telling him, "I just wanted to find you, so you wouldn't be so lonesome" (13.35). When she tries to follow him into Terabithia and gets stuck on the makeshift, tree branch bridge – and really puts her life in jeopardy – saving her is something that brings Jess back to himself and helps him realize more important lessons Leslie taught him. May Belle shows Jess he's still loved and needed even after Leslie's gone, and makes him see he can still get to Terabithia. It seems like he makes a real, proper bridge across to Terabithia so that May Belle can go there with him too. In teaching her to see Terabithia for herself, he's reminding himself of how to see it as well.