Jin Hotaka was once the most powerful and famous assassin in the underworld. One day on a mission, He and his teammates were on a impossible position and his little sister Kaede was fatally injured and was inevitably going to die. He decided to be the good big brother until the end and save everyone and die with his little sister. When he opened his eyes he is in a different world called ""Ryvern"" where he is a reincarnated as a child named Haru Haru who was detested and called a demon's child because of his peculiar and demonic eye called ""Tyaera"", which is said to contain all the darkness of the world. There he decided to adapt to this world and try to search for his sister.