Responses
The blue haired guy was probably keeping an eye on san as he is being targeted by hwan ( the evil ghost guy he is hunting). 704 was in his human life someone who committed a great sin, and that must have involved blue-hair-guy. From what we can see blue guy already knows yama and he is helping with hunting hwan out of some of revenge (presumably?). It also seems that san may have some kind of spiritual power as he could see hwan and blue guy.
I'm trying to understand what the heck happened so let's go;
1-San was a Human okay that I know
2- I didn't get why blue-hair-guy was in San's house BEFORE everything happens (San's still with his aunt - 20 years a go - on the day he met Hwa like wtf?? Like, it seems the same place but with some changes?? Like he's in the future seeing the past? Is that possible?
3- Blue-Guy either fell in love (which is obvious) with 704 or He knew him from before (f*ck u Yama :)