Luke Walker

Sergeant Walker is the American instructor sent to train the four cadets. He accompanies them to Watcher's Nest, continuing their training under the watch and direction of Lyar. Strict but fair, he is ready to drive the cadets well beyond their tolerance if it makes them tougher the next time. He does not consider himself a babysitter, though, and later refuses to train the unmotivated Misaki until she recovers on her own. from wikipedia