I'm currently reading a cyoa, and the protagonist at one point gets possessed by a demon. The demon provides power, but also corrupts, offers dark commentary, is a constant nagging presense, etc. It reminded me of Cyberpunk 2077 and then I realized I want to read a fantasy book based on this premise of "two people one body."
The specifics of how the other entity behaves/how it's personality is like aren't super important as long as:
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The entity and the protagonist interact frequently.
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The entity influences the story.
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The possession (and it doesnt have to be possession!) happens early on, and lasts the majority of the story.
by Impossible-Voice471