i'm so sorry if this is confusing but, i'm trying to find stories where a loved one is replaced. not by something that's outwardly trying to harm them (though i don't mind that to an extent) but, by someone trying really hard to blend in.
the person that's replaced can be an alien, or another human. i don't mind as long as the replaced is not animalistic. one thing that i don't like about a majority of skinwalker stories is that, the moment someone is taken by one, it's extremely obvious. they act feral or have extremely odd behaviors.
i think having a friend or a lover be replaced by something that can blend in so easily is much more terrifying. maybe the people around them actually start to like them more ever since their autonomy was taken. and when the twist is revealed the people around them have to grapple with, not just the loss but, the fact that they never really viewed the original in a good light.
i'm just throwing spit-balls here but, i hope you understand.
i don't have any book examples so, i'm sorry but, here are the ones i've liked:
kira yoshikage as kosaku kawajiri in jojo's bizarre adventure
nolan from invincible*
hikaru from the summer hikaru died (have not read but, the premise is what i'm looking for)
*i understand that nolan is not a replacement story but, seeing the way his life ruined everything for his family, friends, and, the trust of the people on earth because he was living the lie of humanity too well made me very sad but, terrified so, if there are any stories like that, i'm willing to read it too.
by LimpLicker47