In Ender's Game what is Mr Graff like?

Graff is the Princible of Battle School, at least until he got court marshled for the incident with Bonzo. To Ender, Graff seems like any other teacher, trying to use him and break him. But Graff seems to have hidden affection for Ender, and is reluctant to push him to his limit.
P.S. Enders game is awsome