Yes, I think Leslie is a very brave character.  Though she is just a little girl, she is courageous enough to be unapologetically herself.  Many adults are not brave enough to do that, and Leslie does not try to change herself or her appearance in order to be accepted.  She does not fit in: when Jess first sees Leslie she looks like she could be either a boy or a girl, her family does not own a television (the horror!), and even the food she brings for lunch—yogurt— makes her a target.  Yet, Leslie just keeps right on wearing what she wants, doing what she wants, and befriending who she wants.  When she feels as though some injustice has occurred, she steps up to right it: she helps with the plan to get Janice back for taking May Belle's Twinkies, but once she learns how difficult Janice's life is (her father hits her), she changes her behavior and is kind to the well-known bully.  She is not perfect, certainly, but she is a child and a very brave one at that.

Yes, I believe that Leslie is a brave character.  

Leslie's family has recently moved to the area, which means that Leslie is the new girl in school.  That is a scary situation for most kids.  Leslie very easily could have shied away from the other kids and hoped that some of them approached her.  That's not Leslie though.  During the first week of school, she bravely walks up to the clearly defined running races at recess.  Not only is Leslie brave enough to approach the group on her own, but she is also brave enough to approach a group that is completely male.  It has to be obvious to her that the activity is boy only.  Nevertheless, Leslie asks to participate.  Then she wins, and she wins over and over again.  

Another example of Leslie's bravery is when she goes into the bathroom to find out what is wrong with Janice Avery.  Avery is a complete bully, and she could physically or verbally harm Leslie in this situation.  Leslie takes the risk.  

Leslie also bravely crosses the bridge into Terabithia while the stream below is flowing dangerously.  Unfortunately that brave act winds up killing her, but the act was indeed brave.