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Another alternative is to have other characters question the existing of the friend, accuse the MC of making up stuff, and the imaginary character is never there when the MC could prove their exist...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47839 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
Another alternative is to have other characters question the existing of the friend, accuse the MC of making up stuff, and the imaginary character is never there when the MC could prove their existence. I have seen this cut in **both** directions; if the never-there character always has a very plausible excuse for not showing up or for disappearing, they can turn out to be a real person. Or perhaps they just don't want to be seen, because one of her friends might identify them. A place where an imaginary character played a major role and stayed hidden like this is in Mr. Robot, the TV series, in which the MC Elliot is mentally ill, a split personality that has conversations and physical fights with his dead father, becomes him at times, cannot remember his sister IS his sister and at one point kisses her (which she struggles against), and is often losing his grip on reality. But he's a great hacker. In the beginning of the first season, to the audience you can't tell that his dead father is not a real person. But then he also thinks he is imagining people that are in fact real.