I had one so pervasive in high school that everyone thought it was how I actually was, and the few people who didn't wondered what was wrong with me when I suddenly went all horrible. I was sarcastic, mean, and generally bastardly. I even got named (I did not come up with any of these names):
Prince of Darkness, Dark Lord, Lord of Darkness. (I wore a lot of black as well)
Nowadays I slip into a sort of wacky crazy fun-loving type person, life of the party loud and extrovert. It's closer to what I'm like in normal situations, but more of an exaggeration than actually me.