Ballad X Opera summary:

An angel and a demon who're conducting a mission on earth end up killing a human named Haruto by mistake. Haruto, who has no reason to live, accepts death... But for some inexplicable reason, his soul can't go to heaven and he becomes a wandering soul. With a mission to accomplish and the guilt of having killed someone weighing heavily on him, the angel asks Haruto to become a Shinigami and help them out. In exchange, he promises to give Haruto something that will change his ‘life’ and his future forever.