His mind isn't even human. It's dead. Why are the dead the first to rise during psychic threats? Why do Daleks hardwire fresh corpses to their whims more frequently than even normal humans? But I've seen a Priest wield off a horde of vampires. Sgt. Howie was immune to the plague Arthas created. A companion loyal enough to me can create a psychic barrier capable of blocking out even the strongest of otherworldly invaders. He's at least willing to try, and willingly of his own choice and without undo influence agreed to not kill anyone during the progression of his studies.