More than anything, Leslie's defined by her imagination and sense of adventure. She's open and ready for anything: ready to move to a new town and make new friends, to try new things, to create something where nothing existed before. Even though moving to Lark Creek was hard on her, she wanted to do it to support her parents.
Because of her reading and the way she was brought up, Leslie is able to use her imagination and creativity, bolstered by her friendship with Jess, to get outside of Lark Creek. Terabithia is her idea:
"I know" – she was getting excited – "it could be a magic country like Narnia, and the only way you can get in is by swinging across on this enchanted rope." (4.102)
She's the one who sees magic in the ordinary, sees Terabithia where there's just a forest, and is the one who helps Jess see those things too.
We know what Jess's ambitions are, but we don't really know Leslie's. In fact, we never learn as much about her as we do about Jess, or about what Jess thinks about her. We do know she loves stories – reading them, telling them, and making them up. The girl's curious, she genuinely wants to know things, and she's a "beautiful" (3.87), naturally fast runner. But we don't really know what she wants to be when she grows up, or what drives her she's had a hard time, with only "one and one-half friends," Jess and Janice (7.111). In fact, without Jess, we can bet her year in Lark Creek would've been pretty miserable. Her father later thanks Jess "for being such a wonderful friend to her" (12.13), and tells him "she loved [him]" (12.13).
Jess always thinks of Leslie as full of courage when he is afraid. When he asks her if she's frightened of Janice Avery, it's mainly because the thought of her getting frightened is incredible: "he didn't mean it in a daring way, he was just dumbfounded by the idea of Leslie being scared" (7.70). There are two things to take from this. First, Jess has important things to teach Leslie too, like feeling compassion for scary enemies or "predators." He helps her to stretch her mind just as she stretched his. Second, sometimes being too brave can be foolish or tragic. Because Leslie doesn't feel or recognize fear when the creek to Terabithia is overfilling with rushing water, she makes the overconfident leap of trying to cross it by herself.
Leslie changes Jess's life, there's no question about that. Yet even when she's gone, she's still present for Jess and is still helping him grow into a better, more noble person – a person worthy of Terabithia:
It was Leslie who had taken him from the cow pasture into Terabithia and turned him into a king. […] hadn't Leslie, even in Terabithia, tried to push back the walls of his mind and make him see beyond to the shining world – huge and terrible and beautiful and very fragile? (13.59)
Leslie's gift was that she knew that "shining world," in all its terror and glory, joy and heartbreak, was out there, waiting for them, and she was able to "make [Jess] see beyond to" it. We can only imagine what she herself saw and what else she would have been able to learn, and to teach, had she lived.