There was one demon in the universe of Demons and Demon Killers. He was the 2nd highest Moon, and his name was Ken. He had 1 katana. Katana was able to separate the demonic half of a human from the human half. One day he met a girl he fell in love with, her name was Minami. He did not kill her, but helped her cope with her hardships, after which she already fell in love with him. Minami knew Ken was a demon, but she wasn't afraid of him because she loved him. So Ken and Minami lived together in joy. They had a half-demon son, named Arima. Their life was full of kindness. Ken even refused to eat people for the sake of their family.

But on one of the days, Mudzan found out that Ken had started a family with a human woman, although he was a demon himself. Mudzan ordered their family to be killed by sending demons of the higher moons there. Ken hadn't fed on humans for several years and became weak, which is why he couldn't fight back. He was killed with Mines. Arima managed to escape. That's how his long life of loneliness began...