Sylvia is Dally's ex-girlfriend who cheated on him while he was in prison at the beginning of the novel. She does not make an appearance in the movie, but she is mentioned by Dally near the beginning.
In the book, Ponyboy mentions that once, while Dallas was in reform school, Sylvia had started hanging on to Johnny and was sweet-talking him, which the gang didn't like at all. When Steve got a hold of her, he told her that if she tried any of her tricks with Johnny, he would, as Ponyboy put it, "personally beat the tar out of her".
Sylvia's appearance is unknown, but since in the book it is mentioned that Dally (as well as Two-Bit and a couple others) tend to go for greaser girls who wear heavy eye make-up, are tuff, drink or smoke and swear every once in a while, it is possible that Sylvia was one of the greaser girls.