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It isn't actually imaginary, or really friendly,
So what is it actually? A grotesque mass of all the bodies it has ever taken over?
The first instance was an accident,
I find this hard to believe, especially since it apparently just started off as a voice and apparently never befriended anyone at all in its existence, and the one person it happens to become friends with just happens to get assimilated?
it had essentially withdrawal symptoms, it became hooked on being alive. It was now dependent on gathering more bodies to use
Kind of feels like the premise for the Torchwood: Children of Earth series, with this thing essentially being the 456 alien thing that needs children because of the drug-like chemicals they produce. That said, if this thing got hooked on "being alive", why does it go after people in particular as opposed to animals?
I feel like you have a very, very small start on a workable idea, but typically monsters that are the way they are just because of some sort of nature-driven habit tend to fall a bit flat. And the audience has seen a lot of monsters, so it gets increasingly harder to impress the community with setups like this.