The reason why Luca is not acting the same as the original plot isn't because somebody possessed his body. It's because this is the second time he's lived this life.
[SPOILER] the memories from "the novel" that judith has, are actually just luca's memories. Also judith is not a reincarnated FL. She is the real judith and always been. The memories she see's are from her previous life (i.e. the life she lived before judith was born).
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The reason why Luca is not acting the same as the original plot isn't because somebody possessed his body. It's because this is the second time he's lived this life.