You can almost count the number of times that adults tell Ender the truth on the fingers of one hand. Ender sums it up pretty succinctly when he notes, “I've spent my life as someone's pawn" (15.97). Much of this novel is about how adults manipulate Ender in order to fulfill their needs; they trick him and lie to him and tell him just enough so that he can defeat the buggers. Basically, manipulation in Ender’s Game is a gray area. On one hand, it seems wrong to lie to and cheat a kid. On the other hand, the adults manipulate him to ensure the survival of the human race. Does that make it OK?
Besides the manipulative relationship between the adults and Ender, there are a number of other relationships full of manipulation: Peter (sort of?) manipulates Val; Peter and Val manipulate the world; and, frankly, Ender manipulates a bunch of the other kids throughout the book. Manipulation sure sounds negative, but it sure gets the job done.