Ohhhhh

OK FINALLY, so they were actually the characters, they were written into the novel, after they both died they both went into a different timelines AND world.

Ok so this is just a theory but since they both were at different timelines when they came back, while the ml was in the highschool timeline, the Huimin was in the world he believed in, and where the ml was sweet and his friend (?) until he discovered the book again and regressed into the "book" but that wasn't the book he actually read, it was the timeline where ml was already trying to change things.