
It is all one Iheon.
SPOILER
The novel that full of misunderstanding and abuse is actually the real world. Just don't know how, Huimin who live happily with normal parents and friends actually a character that the abused Huimin wrote, like a wishful daydream that things can be just normal and happy. In that happy version of world, characters and surrounding only get to life if Huimin is there. When he's gone, everything become stagnant. Iheon in that happy world released it, but not really questioning what happened to that world as he focus to play his character as a supportive best friend.
Then Huimin from the happy world come to the real world, where the original Huimin living. The timeline go back to the past. At first, Iheon has no memory about the future (Huimin's death) and about the happy version Huimin made on his sketchbook. His memory then come back. He remember all; about the tragedy of him and Huimin, about the world Huimin wrote and they living in there. Iheon now has the memory of all version of him so it can be said Iheon now become one. For the now Iheon, Huimin just lose his memory about their tragedy which is why he try so hard to cover the truth by hiding that sketchbook. If the now Huimin find out about the happy world he's living actually aren't the real world but a daydream from depressed Huimin, his innocent and happiness might shattered.
What about the iheon from other timeline?? :(( hope he gets a happy ending too