Here's some spoilers i've found from someone:
SPOILERS:
Basically, it goes like this:
Hyeonjin starts working as Mujin's underling.
Jaewoo gets sad.
Yoo appears.
They make a deal and then asks Jaewoo to die and return to his OG world WITH him.
Yoo is actually some kind of old friend (OG name is Eunjoo) of Jaewoo's who's super obsessed with him.
So to help Hyeonjin, Yoo and Jaewoo planned out a perfect plan to ensure that his killing/death will get Hyeonjin out of debt.
It happened and everyone was mourning.
Hyeonjin inherited all of Jaewoo's wealth.
Mujin and Hyeonjin later became friends and they often went drinking together.
Back in the OG world, Jaewoo was desperately looking for a way to return.
He begged Eunjoo's sister Eunha for help, she had the same ability as Eunjoo. She told him if he were to go back, he would go as his OG self and that his existence in the real world would just disappear as if he went off-grid.
Boom he arrived back in the novel.
He started working at a restaurant and one day, one of the kids brought food home from that restaurant where Jaewoo works at. Hyunjin tried it and then was reminded of Jaewoo's food, so he ran there and started confronting him finally Jaewoo said "You wouldn't believe me anyway."
But Hyeonjin promised to believe no matter what.
So they reunited happily.
Hyeonjin went often there to eat, but one day when Mujin and Hyeonjin were supposed to meet up, he said no, so Mujin suspected he was cheating on Jaewoo. (Like cheating on Jaewoo, cuz found someone to replace him so fast after he recently passed away).
He went there and saw Hyeonjin looking so much healthier, he demanded answers, but got none, since he wouldn't believe it anyway. He left and later came back with his underlings and destroyed the restaurant where Jaewoo was working.
Finally, Jaewoo gave up and confessed, Mujin of course was shocked. After talking, he left.
After that, Mujin's underlings fixed everything up.
Mujin occasionally visits them, but as a friend, he gives up on pursuing him, saying that maybe one day love will find him too.
So what happened to Yoo?
After returning back to reality, Yoo lost his power, hence he can't go back to the novel world. The end. ٩(๑❛ᴗ❛๑)۶
Or we can assume that since her sister sent Jaewoo back to the novel world (she said he won't be remembered anymore), maybe Yoo forgot about him too or not. Who knows. He wasn't never mentioned again after Jeawoo returned.
his sister*
Givesome spoilers plzz