In Stargirl by Jerry Spinelli, what is the one thing about Stargirl that makes her different?

If I was forced to pick a single character trait about Stargirl that somehow manages to encompass how amazing she is and explains why she is different from other students, I would go with the character trait of self-confidence. 
Self-confidence is a feeling of trust in your own abilities, qualities, and judgment.  Stargirl has a lot of self-confidence.  She likes who she is, and she doesn't feel the need to transform herself into something else for the approval of others.  She wears the clothes that she wants to wear regardless of fashion trends, she sings "Happy Birthday" to people that are complete strangers, and she cheers for both athletic teams with equal fervor.  Other people find Stargirl's attitude and behavior odd and even off-putting, but Stargirl simply does not care what they think.  She is confident in herself and all of her idiosyncrasies.