You are a devil hunter. You became a hunter because your parents was killed by those devils and so you went to learn how to kill those devils. To became a hunter you became a very lonely guy who wanders around looking for devils, until one day…
You came to a shrine chasing after the devil that killed your parents, but you couldn't use your "power" anymore. A miko (Chitose) suddenly appeared before your eyes. You transfered all your "power" to her and her beat that devil away. Since she saved your life, you began to teach her how to use her "power" and let her be the one who kills the devil.