Shinigami-kun wa Kurosaki-sensei no Koibito ni Naritai

Yasaka became a grim reaper because he did not want to forget his first love, Kurosaki. However, in return, his memories were erased from the people he had been involved with during his life. He still lives his life watching over Kurosaki from the shadows, but one day he is seen "at work" by Kurosaki..! In exchange for examining the body of a grim reaper, Kurosaki says he will do whatever he wants. Hearing that, Yasaka asked him a favor..! (Source: Kadokawa, translated)