Dori Dilson is the friend everyone wishes she had.
Unfortunately, other than this tidbit, we don't know much about Dori at all. That figures, of course, because Leo is our narrator, and he doesn't actually know Dori. Only Stargirl does.
From Leo we learn that Dori is a "brown-haired ninth grader who wrote poems" (8.13). She is the first to be brave enough to sit with Stargirl at lunch, which opens up the floodgates for everyone else to sit with her on her rise to popularity. She is also the person who staunchly refuses to shun Stargirl. That tells us that Dori is majorly loyal. She applauds for Stargirl's songs, and she's the only one to sign up for our girl's group, the Ukee Dooks.
It is a shame that we don't get to know Dori more because she seems just plain awesome. In fact, you could argue that she is the bravest person in the novel. Yes, Stargirl is brave, but in a way, Stargirl is so far beyond caring what anyone thinks, that for her it doesn't take much effort at all to be herself. Dori, however, is one of "them," and she was never very popular to begin with, so she must feel the sting of Stargirl's shunning. Through all of this, though, she never wavers in her devotion to her friend. In fact, the only time she ever shows any disfavor toward our girl Stargirl is when she turns into Susan. But even then, that's just because Dori, wise young thing that she is, knows that she has, in a way, betrayed herself.
Having a true friend like Dori is rare, and serves to remind Stargirl, and Leo, too, that when it comes to friendship, quality trumps quantity.