He and he were originally in different worlds. An accident caused them to meet and start their adventure...
He has no name, he is a vampire, but he has abilities and characteristics that are completely different from other tribesmen, so he was rejected. He came to that person's world during a family war, and he can never leave him again. There is only that person in his world.
He never has limits in what he does, and everything follows instinct. An encounter determines his fate and life, gives him a name, and becomes his salvation...