How is Stargirl different from other girls?

With a character named Stargirl, a reader could only hope for someone fabulously captivating—and that is exactly what Jerry Spinelli delivers in this novel. But to contrast Stargirl with "other girls" would require putting all "other girls" in a box together, and that isn't really possible. Stargirl would probably not even be cool with that herself. So let's instead look at how Stargirl is one of the most individualistic characters of all time, regardless of gender.
Stargirl is compassionate beyond measure. The girl sends thoughtful gifts anonymously to unsuspecting people just to be kind. She cheers when a kid on the other team makes a basket. She attends the funeral of a stranger. Stargirl doesn't just make a halfhearted effort to spread love to the world; she really engages with people on a deep level. She seems to feel both the world's collective pain and its joy and takes active measures to connect with people in both situations. Sadly, many of her efforts only create anger and resentment at her school, isolating her from the very people she cheers on. Which brings us to another unique personality characteristic of Stargirl:
She really and truly doesn't seem to care what people (collectively) think about her quirkiness. Lots of people lay claim to this feeling, but as a whole, people dress and act a certain way because society and/or the group they long to be a part of have particular expectations for these behaviors. Stargirl dresses as she wants. She befriends whom she wants. She doesn't seem to care about the expected social norms at her high school (even cheering for people who stop to pick up litter) and when people get mad at Stargirl for acting very much like Stargirl, Leo notes that "she did not seem to notice" (chapter 11).
One more characteristic distinguishing Stargirl from the masses is that she seems to view the world in endless beauty and full of fascinating possibilities. She takes time to note the struggle of ants. She watches a lone man sitting on a park bench and dances in the rain. Stargirl is a true free spirit who cannot be easily classified.
Stargirl represents one of those characters everyone longs to meet in real life—someone who is passionate, loyal, and forever concerned with the welfare of the universe. Her unique personality makes her one of literature's most unforgettable characters.

Stargirl isn't only different from other girls. Stargirl is different from everybody. I would say the main character trait that makes her stand out from everybody else is her self-confidence. She simply does not care what other people think. That is how she is most definitely different from other high school girls her age. Stargirl does not care about fashion trends. She wears what she wants to wear regardless of what is in fashion. She does not play favorites, as illustrated by her cheering for both sports teams and singing Happy Birthday to strangers. Additionally, Stargirl most definitely does not spread catty rumors about people the way Hillari Kimble does. Stargirl embodies love. She loves everybody equally, and she loves them even when they don't love her in return. That's how she is different.