The Enders Game
By: Orson Scott Card
Ender is selected by international military forces to save the world from destruction. Ender is made commander of Dragon Army and given a group of soldiers, most of them Launchies. Ender gets into a fight and he hurts four older boys who attack him. It is clear to Ender that the teachers are leaving him to fend for himself. Ender's army is given an unprecedented number of battles to face, with advantages given to the enemies, and he wins every one.
Ender is the youngest of the three Wiggin children. Ender always wishes for events to be resolved without violence. His compassion is his strongest trait.
Valentine and Ender's older brother, Peter is completely lacking in the compassion that the other two have and has only ruthlessness in its place.
Valentine is Ender's older sister, does what she can to protect him from Peter, their sadistic older brother. Valentine has much of Ender's compassion, but she does not have Peter's ruthlessness.