Only You in This Mortal World Are Worthy of My Love

Without you, what use would I have in this world? Without you, why would I love this world? A bad boy who delivered letters in ancient times strayed into the future and traveled through the castle. He was randomly assembled and thrown out to wander around the world. To find a relative! For revenge! For the love of your life! Technology versus demons, ghosts, immortals, Buddhas, humans and gods.