I don't believe there is a character by the name of "Cindy" in The Outsiders, but you may be referring to Sandy, who is Sodapop's girlfriend in the story. (There is also the mention of a Sylvia, who is a former girlfriend of Dallas Winston.) Soda and Sandy often double-date with Steve Randle and his girlfriend, Evie. Soda hopes to marry Sandy when she gets out of school. However, Sandy eventually becomes pregnant--apparently not by Soda--and she is sent to Florida by her parents to have the baby (and possibly be away from Soda and/or the actual father). Soda nevertheless proposes to Sandy; however, the letters he writes to her are returned to sender, leaving Soda heartbroken,
Sandy is sodapop's girlfriend that got pregnant by a other man that isn't sodapop. After she got pregnant, she moved to Florida to live with her grandmother. Soda writen her a letter that she returned unopened.
Sandy is Sodapop(Ponyboy's brother)'s girlfriend. In the beginning of the book, Soda talks about how he wants to marry Sandy. But then in the end of the book, Sandy apparently becomes pregnant with a child that isn't Sodapop's and gets sent to Florida to live with her grandmother by her parents. We disover this when Darry and Ponyboy are fighting and Soda gets upset and runs out of the house into the empty lot crying. Darry tells Ponyboy about Sandy and then Pony and Darry go outside to talk to Soda. Sodapop tells them about how he wishes that they wouldn't fight anymore, and Darry and Ponyboy promise to never fight again (or at least to try not to).