David Cain is one of the world's premier assassins, whose victims have included some of the most famous and powerful people on the planet. He trained the young Bruce Wayne with some of the skills that he would use as Batman, although Bruce has never used some of the more lethal techniques Cain taught him and has since surpassed his teacher in ability. Regarding his decision to train with an assassin, Batman explained, "Knowing "how" to kill doesn't mean you "must" kill".