Someone who tries the rebellious teenager image even though they were always the goody two shoes who behaved exactly within the rules as expected by authority figures, and got other people in trouble or spied on them as a kid.
The poser was never any of what he/she claimed to be.
You can tell a poser imitating a kind hearted person from a kind hearted person, since a poser wont pose as kindhearted without also adding something about themselves being badass to their claim of being kindhearted.
The poser would pose as kindhearted when he/she wanted acceptance from the kindhearted folks, and as an asshole when he/she wanted acceptance from assholes.
Someone who thinks they're making a major statement or changing the world every time they do something an asshole would do, like it's their first time being that way and they have to do/outdo more to prove they are an asshole to other assholes than everybody else.
The poser thought everything he/she said that an asshole would say needed to end up as a quote online somewhere, otherwise people would remember what he/she was like growing up.
Someone who is really everything they try to make fun of, while imitating everything they are while trying to make fun of somebody.
The poser had to wait on somebody else to come up with material so she would have something to say was not theirs originally.
If a guy has convinced people he's Italian (such as an actor in a movie), but he's not really Italian, no matter how believable the guy is, he's not authentic, so he's a poser.
The poser had convinced people that he/she was from hell and that he/she was hell, but since there was no hell, he/she was just another otherwise obscure poser.
Someone who is not who they say they are.
Even if he/she filed his/her teeth down into serrated edges and wore black contacts, he/she still wouldn't have anybody convinced that he/she was really a shark. He/she was a poser, posing as a man eating denizen of the seas.
Someone who's not who they say they are, or who they appear to be.
He/she was a poser because the person he/she was posing as might actually be missed by a few people.