Who is the spy for the greasers in The Outsiders?

The spy in Ponyboy’s group is the Soc girl that Ponyboy befriended: Cherry Valence.

At the drive-in movie, Johnny and Ponyboy meet a couple of nice girls that just happen to be Socs.  As greasers, there is a big class and gang difference between the two groups, but Cherry is impressed with Ponyboy’s intelligence and sensitivity.  They become friends, and Ponyboy realizes that Cherry is not so unlike him.

Unfortunately, Cherry’s boyfriend is one of the Socs that Johnny and Ponyboy fight with in the park.  The boys’ meeting with Cherry sets off a terrible chain of events.  When Johnny and Ponyboy have to run away, Dally brings them news of Cherry’s defection.

Hey, I didn't tell you we got us a spy."

"A spy?" Johnny looked up from his banana split. "Who?"

"That good-lookin' broad I tried to pick up that night you killed the Soc. The redhead, Cherry what's-her-name." (ch 5, p. 84)

Ponyboy is surprised to learn that Cherry would come to the greasers with information.

"She said she felt that the whole mess was her fault, which it is, and that she'd keep up with what was comin' off with the Socs in the rumble and would testify that the Socs were drunk and looking for a fight and that you fought back in self-defense." (ch 5, p. 86)

Ponyboy notes that it was not the Soc part of Cherry that was helping them, but the “the dreamer who watched sunsets and couldn't stand fights” (ch 5, p. 86).  He realizes that Cherry is just trying to right a wrong.

Cherry’s providing information and becoming a “spy” for the greasers demonstrates the hopelessness of the situation.  Cherry has had it with the senseless violence.  Her boyfriend Bob was killed, and she knows that more boys on both sides are at risk.  She hopes that by warning Dally she can prevent further bloodshed.  Now that she has met Ponyboy and Johnny, she knows that greasers are people too.