From what I recall San has a lot of energy (not sure if this the name) that the monster can be fed. Like a lot. As never seem before.
Geumsun was the goblin that first hurt the monster long time ago and now he's trying to get the same monster for good this time.
704 was just a poor reaper that was caught in this mess. The goblin and the leader from that part of the underworld (don't remember his name) kinda made 704 make the mistake and take San to the underworld so he could be disguised as a goblin.
Now they're going to use San as lure for the monster so the goblin can take the monster down. But, I'm not sure if San and Eunjae know about this plan.
Y did san loose his memories?
After losing his aunt, the monster came after him and the goblin saved him, but San was too devastated with his loss, so ha made kinda a deal with the goblin. He was going to follow the goblin's orders in exchange all his memories will be wiped out, and will only have them returned once he asks for them back.
I'm still confused. Of the story why is san ? Why is geumsun? And why is 704?? Could someone explain what is happening