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.