This might be a spoiler but this is just my idea. I feel like the og female lead is the book, like her spirit is that of the book or the person who keeps possessing others to help her. Kinda like a system in other stories, now the question is, who brought her to the book and why.
I think it was a god or a high spirit but , since there's no clue as to what happened to the og female lead it could've been she asked god to give her a new life. Grampa could've been possessed by "God" to give her clues that aren't real. Our female lead is like a guide for the crown prince, she protects him and more than likely will stay with him. He already feels jealous of his guard captain. I dunno, take it as you will this is all I've deduced from clues and little info, I've not read the novel if there is one.
this has sooooo many layers into it and im kimda excited for whats coming like, where is a og fl ? anwww i hope this dont disappointtt