General Antonyms:
* Legitimate: This implies someone who is genuine and has a right to be where they are.
* Authentic: This suggests someone is real and not a fake.
* Genuine: Similar to authentic, this refers to someone or something being true and not a copy or imitation.
* Real: This is the most basic antonym, simply meaning the opposite of fake.
More Specific Antonyms:
* True heir: If the imposter is trying to take the place of a rightful inheritor.
* rightful owner: If the imposter is trying to claim ownership of something that isn't theirs.
* Valid authority: If the imposter is pretending to have a position of power.
The best antonym will depend on the specific situation and the meaning you want to convey.