Hwan - ghostly monster ; not a human ; not a ghost. They're a natural enemy of a Reaper. And Hwan has been the most troublesome obstacle that Underworld has ever experienced. He preys on humans to get energy and to get stronger, but, in order for him to be successful, he has to take care of those bothersome reapers and goblins that tries to block his way.
His new target was Joo EunJae, because he has a strong energy compared to any humans. In order to prey on his soul, he has to take care of those people around him or what they call emotional support ; thus his parents and his dog Hojungie.. He killed them off, and if a Ghostly Monster ; or Hwan killed a human, it will come as "Unknown" to the Underworld's Death Record.
It was his original plan to create chaos to the world of Underworld, to make Reapers busy and weak. That's why he keeps on killing and preying off humans.
Yama - an Underworld King, however, has it all wrapped up. They have a plan. He sent Geumeun ; a Goblin and one Reaper ; 704 to look out for San (a human before that became a Goblin out of will) and Eunjae.
Eunjae was Hwan's new prey. And they're planning to block him and catch Hwan again for the last time, thru San.
On San's story....
His family and ancestors were known for spiritual activities and knowledge. His mom and his aunt can feel and see ghosts too. Like him. What happened to Eunjae, has also happened to San. He was once a HUMAN, Hwan's prey, and therefore, in order for San to be weakened, he eliminated every San's support ; his mom and his aunt Sul. However, San wasn't easily weakened. And Geumeun was able to aid on to him before Hwan get to prey on him. Geumeun was able to slash thru Hwan's throat and therefore loses his strength.
And granted one wish to San, to be resurrected as a Goblin, but not entirely as a Goblin. He's still a human , but resurrected as one. And his memories are all wiped out by Geum. He accepted the offer that ONCE they saw HWAN again, San will be the VESSEL ; a sacrifice.
When San met Eunjae, Eunjae was already possessed by Hwan that day. He was close to death. His records was already in the Underworld's Death Recods. But, 704 was able to touch him and prevent him from dying. But that doesn't mean , he's alive. He's only alive because of his soul but his body seems to get weakened and considered dead.
That's also the same with San. He never really existed.
When Yama and Geum was now ready to capture Hwan... San has finally regain his memories and created an opening to capture Hwan on his own body. And Yama and Geum's plan are already taken to its place. However, Eunjae's memories of San are wiped out.
San sacrificed everything for Eunjae's sake :((
Fate Makes No Mistakes