The unexpected spy for the Greasers is Cherry Valence.
The reader is introduced to Cherry in chapter 2. She is hanging out at the movies, and over the course of the chapter, she and Ponyboy essentially become confidants in each other. Ponyboy tells her about Johnny, and she tells Pony that all Socs aren't like Bob (her boyfriend). She also tells Ponyboy that "things are rough all over." It's the first time that Ponyboy has considered the Socs as people.
After Bob's death, Cherry becomes a spy for the Greasers.
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."
The reason that Cherry gives to the Greasers is this:
"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."
Ponyboy knows that Cherry's reasons run deeper than her professed reason. Ponyboy knows that, like him, she is a dreamer that wants to see the world become a better place. A place that is not filled with senseless violence. A place that doesn't have hate and discrimination based on socioeconomic differences. Cherry becomes a spy, because she believes that she can help stop boys on both sides from getting hurt.