Ender changes constantly throughout the book. His changing isn't a nice, smooth, continual path of positive change either. He sometimes reverts back to how he was before, or changes to be like the very things he hates.
Ender is the sibling of Peter and Valentine Wiggin, and he shares Valentine's compassion and Peter's ruthlessness. For example, Ender always wishes to find a peaceful end to brewing conflicts, but he isn't opposed to using violence, either. This is illustrated when Ender, as a young child, beats Stilson to death in order to protect himself.
Ender also goes through multiple changes with his relationship toward Graff. Ender first believes Graff is there to protect and help him. Ender then realizes Graff has been manipulating him from the very beginning. At one point, Ender shuts down completely because of this, but he then returns to the space station willing to deal with Graff and his games.
As the story nears its conclusion, we see Ender finally realizes he can't save everybody. This is a sad change in Ender because readers see him begin to manipulate friends (like Bean) like Graff does to Ender. As the novel concludes, readers see Ender manages to hang on to his humanity. He takes it upon himself to find a new home world for the buggers. This is his way of repaying the debt he feels he owes for almost completely eradicating them from the universe.