Spoilers below:
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So the book FL read when young is not actually the original story. In the original story while Elincia is the protagonist, FL also doesn't have a bad ending. They all have a happily ever after. But a fan didn't like that. She felt FL didn't deserve it.
So the fan wrote a fanfiction of the original story which was the book FL read. The current Elincia is that fan transmigrated into Eleincia's body and yes she's evil.
This is also why I don't hate the Emperor as much. Because what he did to FL in the book (the fanfiction) was not what was gonna happen. In the actual story, the Emperor was quite good to the FL even after it is exposed she's not his daughter iirc. Or at least he didn't kill her off or anything.
Woah.. What a twist! Thank you so much dear (づ￣ ³￣)づ
Spoilers please (●'◡'●)ﾉ