Spoiler!!! And some of my theories...
Here's my Theory:
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;hyuimin the dead one
;min is the possessor
;iheon is the traveler
;iheon the friend(hyuimin possessors friend)
The real "hyumin" is dead but the memories remains or maybe the min is switched with the possessors currently life like alternative world if not...
Then this story has more INSANE plot twist like:
*Hyuimin and min are the same person existing but with really multiple personalities due so many happened to him as we can see he killxd himself right? That shows in the hypnosis therapy hyumin is a traveler too but has MPD
As for iheon he traveled back where it ends he died and just like him he knows everything because he killxd himself too and that's why he knows the future too.
*Iheon is the friend of hyuimin(possessor) he died in a plane crash... When he was about to leave also hyuimin got the news and ooffed himself too because he realized he loved him but when he got in the book he forgets it but iheon doesn't remember it too that plane crash or maybe... Who knows how this manga works might be bigger plot twist
I feel like this ain't no novel.
I think this is like they both dead just like hiumin seeing the memories. But only Ilheon remember the past and hiumin doesn't. He did say he'll protect Ilheon and he doing it. *here I'm reading another similar story "BEYOND MEMORIES"