God bless you man you just saved half the readers here with your explanation
From what I understand on the scribid translated chapters (spoilers ahead)...
He starts to appreciate and wants to stay with the Ghost created by Muni the snake (the one that was a part of a scam to get energy and to spy on Giyoon, like a mirror of Il in, I think he is a part of the new reincarnated soul)...but...(spoilers ahead)... ... he gets destroyed by Il-in because he is jealous ( chalter 100 and something). I think he will stay with someone because he is as twisted as the ghosts at the beginning, he even shows the fact that he knew and remembered everything but acted as if he "didnt" see it, so that he would still be close to his crush. I really liked this side of him, it made him just as twisted, but then the author changes direction abd majes him more innocent.
The new jealous partner (who worked in the office) is someone that Minu possessed, because he "sympathized" with his unrequited love for the new Il- in reincarnation, that's why he pushed the reincarnation (Jung Yintae), to look at him and not at Giyoon (who Minu thought was a cheap imitation of himself). Especially since he thought since they where alike in their unrequited love, in his mind, maybe he could possess him fully and live out his fantasy through him (he specially hated that the new reincarnation wanted to go out with Giyoon instead, at the very beginning). The Il-in wanted the corpse version not the reall Minu, because he didnt talk and didnt "follow" him around (basically an asshole who once he is ignored wants a piece of attention), so that's why he wants Giyoon, because Giyoon doesn't want him, because wants the current reincarnation (or the ghost that got destroyed).
I think i'll write myself here to conclude what I understand from rereading twice in both languages I understand.
Il-in was an ungrateful young master bastard who was annoyed by Muni, his loyal servant's present. So he decided he'd give him a lesson by putting him into the big snake hole. But Muni was dying because he can't be saved in time after being drowned for so long. The big crap young master realised he lost his toy so Il-in made a dark sacrifaction that involved Yuran's ancestor and the mythical snake that was rumored to nest in that big hole where Muni was drowning from heavy rain water. But it didn't save muni, somehow all of them, including the people who helped with the ritual ended up dead. Except for Giyun (muni's half soul) and Yuran, other didn't end up reincarnated and become ghosts.
So in the modern timeline, Il-in soul that is still trapped inside Gitae's body which however, ended up actually to be Il-in's reincarnation body. But since Gitae's fate is like an empty shell, he can't hold into Il-in's soul. But it didn't matter to him, since he finally met with Muni's half soul that was reincarnated as that sick twink, pardon forgot his name.
But then here the twist, he also discovered that Muni is also reincarnated into other person. Then he realises by Giyun's personality traits that he likes Giyun's version of Muni's reincarnation rather than the other one.
Let's put aside Yuran's backstory, basically she's just the shaman from the past who happened to reincarnated and forgot her past so she thought she has a duty to cure the curse of generations and was offered by the sick twink to help getting rid of it by helping him to go with Il-in's plan to reunit both half soul into one.
Along the way, Il-in learned that the snake trapped in both Muni and Giyun and gave them power. Il-in didn't like this, he secretly planning himself that he'll extract Giyun's and the twink's soul, reunited them and getting rid of Muni's annoying traits and also the demon snake. Like he did in the past. But Giyun knows about this plan as well because the ghost that was created to strengthen the empty shell, Gitae, snitched and took Giyun's side.
What about Giyun's motive here? After learning about the sacrifaction, the past and the soul splitting and a power he holds, he decides he'll be the one who get rid all of them, which let me be detail: Il-in, his half soul, the demon snake, Yuran (the shaman reincarnation), the revenge ghosts and even the ghost that filled Gitae's empty body.
So, now here comes the question. Who the hell am i to supposed to guess he'll end up with? But I wish he'll be left alone to live in peace once it's all done. Well that's all from me, I'll see my way out.