Who is Stargirl? Unlike Leo, who identifies as an American, an Arizonan, and a Mica High student, our Stargirl just won't be boxed in. Unlike many of the people around her, who function well as members of groups, teams, cliques, and so on, Stargirl thrives on her own, free of labels and social expectations. She is, in a word, herself. Maybe that's what makes her identity so hard to define.
All Mica High students, even Stargirl, have no control over their identities. They are simply reflections of their genetics, environment, and other people around them, no matter how much they pretend to be different.
Stargirl shows us that we are in complete control of our identities. We choose who we are, no matter how much other people try to change us.