While some people might argue that Leslie could also fit into the role of protagonist, we see her more as a guide/mentor to Jess. Bridge to Terabithia is the story of their friendship, but it starts and ends with Jess. Leslie appears and expands Jess's worldview, mind, and spirit. She teaches him things that are both ordinary and extraordinary, and also gives him the opportunity to become a teacher himself. Because, while Leslie is such a powerful guide to Jess, she's also just another young kid, although definitely a special one. Sure, she doesn't know everything, she participates in getting revenge on people, and she's too courageous for her own good. In short, she's not perfect. She doesn't finish her job of being a mentor and guide, but maybe that's an essential thing that lots of guides end up doing on purpose: you have to know when to "release" a student and send him or her down the path alone.