Whom can I be?
You can be any character you wish, from any time, place, dimension, or mode of thought.
With that said, there are some practical limits:
The Monolith from 2001: A Space Odyssey would be mysterious, but hard to play; it might be tough to go to the bathroom.
The Abstract Personification of the Electoral College is good for a joke, but the joke might grow stale quickly.
You can play an anti-social character like Freddy Krueger from the Nightmare on Elm Street movies, but your character won't be able to "kill" other characters.
What do I have to do?
Please give me a good enough description so that I know your character's background and personality. I need this as soon as possible; the sooner the better.
I stopped watching TV about 5 years ago; my knowledge of genre culture ends in the 1980's. I'm ignorant about anime, video games, and graphics novels. You're better off describing too much than too little.
Before the event ten years ago, a guest told me he was coming as Roland the Gunslinger from Stephen King's Dark Tower novels. That was all he told me; I'd never read any of those books. I guessed Roland's personality, and it turned out I guessed wrong.
Do I have to play a character?
No, you can come as yourself.