What does Valentines letter mean to Ender in chapter 9

Valentine is commissioned by Graff to try to help Ender, though he is unsure what she can do. He tells her about the fantasy game and Peter's picture, but Valentine does not understand why Peter's picture would show up--she thinks Ender and Peter are nothing alike, since Peter is evil while Ender is good and would never hurt anyone. Graff suggests that Peter and Ender are more alike than she realizes. She says that she often reassured Ender that he was not like Peter, so Graff suggests that she write to Ender to reassure him once again. She does, and later she regrets selling out her brother, despite receiving a high civilian award for her service.

Ender receives the letter, which includes signals proving that it is really Valentine's writing. But he grows upset when he realizes that Valentine has become "one of them now," agreeing to the adults' plan. He perceives that the adults now know all about his feelings about being like Peter and about the mirror in the game.