Formal:
* Mimic: Someone who imitates another person's appearance, voice, or mannerisms.
* Masquerader: Someone who pretends to be someone else, often for deceptive purposes.
* Counterfeit: This usually refers to imitating something to deceive people, like a fake document or artwork.
* Pretender: One who claims a position or role that is not rightfully theirs.
Informal:
* Copycat: Someone who imitates another person, especially in a way that is annoying or disrespectful.
* Look-alike: Someone who resembles another person in appearance.
* Sound-alike: Someone who resembles another person in voice.
* Double: Someone who is physically similar enough to another person to stand in for them.
The best word to use depends on the specific context and the type of impersonation being described.