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Spoiler*
Some people said they are not in the book, all of them happened and huimin (idk if i write correctly) has a multiple personality to cope this traumas. And the last episodes iheon asked what the huimin would do if someone came from the future. So i believe both of them came future because huimin also knows whats gonna happen but i believe he creates "being in a book" as a coping mechanism while iheon really regretted the things he did in the future, so he tries to be better.
In addition, iheon really came from the future becauae in the last episodes he reassured nothing huimin thinks is gonna happen, like he already knew what huimin thinks and what huimin is afraid of.
Also, i realized that iheon gets happy when huimin remembers somethings they did together. So, even huimin didnt say "iheon and i did these, or iheon told me eating alone makes lonely" still iheon gets happy. And that cause iheon thinks that huimin still remembers the moments they share.
Butttt, there is something. In the end, ihron did all of awful things like rape to huimin... i read the first episodes and everything that iheon did was so bad that i doubt that i can forgive him and i also curious whether huimin can forgive him either when he realized all of these happened to him...