Stargirl changes for Leo. A good place to start looking for information supporting this is chapter 23. chapter 23 sees Leo really starting to feel the repercussions of hanging out with Stargirl. He is being shunned just about as much as Stargirl is being shunned, so he starts to avoid her too. The two characters finally get together and talk in chapter 25. Leo keeps saying things like "nobody" likes her, and he tries to blame everything on a hypothetical "them":

"Because nobody likes you," I said. "That's why. Nobody likes you."

"Nobody?" she said. Her eyes covered me like the sky. "Nobody?"

I tried to play dumb, but that wasn't working. "Hey," I said, "don't look at
me. We're talking about them. Them."

Stargirl isn't stupid. She knows that Leo sees himself as a part of the whole. He is partly "them," and Stargirl wants to make Leo and herself happy. She doesn't care what the other people think. She never has. She transforms herself into Susan, and she does it for Leo and his happiness. Archie confirms that for readers and Leo in chapter 32:

"She did it for you, you know."

"What?"

"Gave up her self, for a while there. She loved you that much. What an incredibly lucky kid you were."