I'm a fairly compassionate person in real life, but in games, I'm usually a huge dick. It's very cathartic, stress-relieving, and a good way to get my negative feelings out of my system. The problem is that most choice-driven games don't provide the option to do what I would actually do were I in the character's situation, so sometimes I'm stuck doing obviously craven or idiotic things just because the other choices are even dumber.
In terms of physical appearance, I am a pale-faced, 148-pound college student with stupid-looking hair. I never make the character look like myself, much less act like myself.