Sungyeol is a demon sent to earth by his father Satan, along with some fellow sibling demons, Jieun, Joon, Jaejoong and Hara to chase off guardian angels, or angels in general. There's a rule; you cannot fall in love, or else the person you fall in love with will burn, and rot in hell. Also the demon should make friends, but that's irrelevant.

This is a letter that a 18 year old patient wrote to his ex-girlfriend.