How did Petra help Ender? How is Petra's friendship a hindrance to Ender?

Petra is one of the only people who is nice to Ender despite his reputation. When Ender meets up with Petra outside the battleroom after breakfast the next day, they talk about the mechanics of the battlerooms and the fact that the teachers do not tell the children everything. The realization that the teachers lie about some things brings Ender to the conclusion that the other armies and the other children are not the real enemy; the teachers are. Petra teaches Ender how to shoot. I think these two have a real connection. It's not exactly a love connection but they seem sympathetic to one another. The friendship will hinder Ender when he goes up against her team in battle. Friendships are tough to maintain in such a competitive environment.