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1. Iheon (20s) must've been the first one to get transmigrate inside the novel and has been living a loop since Iheon (20s) and Iheon (30s) likes has been overlapping as we could see from Huimin (20s) perspective. Now that Huimin (20s) is now here, he might be the catalyst for the time looping of Iheon.
2. The novel that Huimin (20s) had read might be their parallel life that they get to live as well before coming to the world (where they are just normal people without any tragic background, where Iheon (20s) is just rich lol) that they now live in. They might not have remembered it but now that Huimin (20s) have read the novel, he's living on it again because he transmigrated inside the novel.
3. Iheon and Huimin (30s) is stuck in a time loop and everytime they both die, Iheon was the only one who could remember it. Huimin (20s) might also be the catalyst to break it since he came from a parallel world.
4. This story might've been just Huimin (30s) remembering his first life and had just committed to that personality because he'd probably break if he remembered the other lives he had or he just really don't remember it, only Iheon (30s) remembers everything.
I think the biggest clue is Huimin (20s) memory of Iheon (20s) where he talks about fate and covering up the truth also the last panel of chapter 29.