Ender is a classic dynamic character in that he changes very much from the beginning of the story to the end. Life in Battle School and Command School hardens him.
At the beginning of the story he is a very young, impressionable, mostly gentle, and empathetic young boy. We see him stand up to the bully Stilson at school, and though he defends himself and wins, he cries at the end because he didn't like having to hurt him.
The same is true on the launch into Battle School. Ender keeps to himself. He observes, and simply wishes to be left alone. He wants the other children to like him. When one boy begins to pick on him from behind, he again defends himself, but later feels guilty about it.
By the end of the story, however, Ender has come to fully accept his place as "above" the others. He knows that he is the best and the smartest. He assumes a commanding position and realizes that he cannot have a normal childhood, nor even normal relationships with others his age. He is confident in his leadership abilities at Battle School with his Dragon Army, and leads them to nothing but victories. He gains confidence (and apathy) with the teachers and begins to talk back to them when the rules of the game seem to constantly change.
When he gets to Command School, it is as if decades have gone by, rather than a short number of years. He speaks to his mentor (Rackham) like another adult. He takes everything so seriously he hardly knows when people are joking with him. He internalizes all of his stress and focuses it on winning.
At one point when he returns to Earth, his own sister, the one who loves him the most in the entire world, doesn't even recognize him.