In chaper 14 Ender is the leader of a group of fighters. His team includes Petra. She is smart, strong, and as Ender characterizes her "brilliant. He had depended too much on Petra and pushed her harder than any of the others. During one of the battles Petra takes her force out too far and looses sight of the others. She makes a mistake, looses most of her fighters and falls apart. As they carry her out of the battle room Ender can hear Petra screaming,
"Tell him Im sorry, I was just so tired, I couldn't think, that was all, tell Ender I'm sorry." pg 199
We later learn, when Mazer speaks to Ender, that Petra will recover.