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

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.