How did Petra help Ender?

Petra is one of the few female characters in the book, and perhaps the only girl among the trainees. Ender meets Petra outside the battleroom at breakfast one day and they talk about adults, teachers, and tactics. Petra helps understand the motivations of instructors and higher up commanders like Graff. Ender realizes that they are pawns, and that their instructors are their real enemies. Petra also teaches Ender how to shoot more effectively.