So that's what happens when an isekaid' person possesses a character's body
Even though we don't see it that much in manga (The One Within The Villainess being a notable exception, albeit beginning with the consciousness returning), it happens a fair bit in webnovels, including ideas like the soul being ejected and only returning if
somebody notices it's not you (And worse, it's called "God's Prank"), coming back after a few years with complete loss of that time's memory, the return of the original character, and them
hating it, and... Crap, there was a manhua one, but I forget the title, where the evil FL is actually the soul of a fan author. Specifically, a darkfic author, and the FL's soul... Is still in there, horrified at what's being done in her name, although it's not clear for, well, the vast majority of the story.
Part of why I like the subgenre, honestly. They play with it a whole bunch, it's like their personal bolognese recipe or something.