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Something about the character's physical appearance doesn't seem quite right. If I recall correctly, the hallucinations in A Beautiful Mind never aged - that's how the MC was eventually able to te...
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#4: Attribution notice removed
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47835 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
Something about the character's physical appearance doesn't seem quite right. If I recall correctly, the hallucinations in _A Beautiful Mind_ never aged - that's how the MC was eventually able to tell that they weren't real people. You could do something similar, to show that the character is out of touch with reality. You could have your MC and her imaginary friend get caught out in some bad weather, and the imaginary friend doesn't seem bothered at all by the wind and rain. Maybe the imaginary friend always wears the exact same outfit, regardless of the weather. And maybe, after a while, the reader will notice that the imaginary friend never needs to eat, sleep, or take bathroom breaks (or have any other needs of her own that might make her unavailable when the MC needs her).