Long brown hair that goes a little below the shoulders, a blue t-shirt or a t-shirt with some sort of gaming slogan or icon on it (A 1-up mushroom, the Half-Life or Half-Life 2 logo, etc), a black leather jacket, and either a pair of blue jeans or grey cammoflage-patern cargos, plus a pair of trainers.
He'd be a sarcastic, jaded and serious misanthrope that would not hesitate to berate the other survivors when they made dumb mistakes or asked stupid questions, yet would also have a habit of being overly-apologetic (to the extent where it, in itself, might become a little annoying). He'd have a very 'I did what had to be done' reason for a lot of his actions, but at the same time, he wouldn't hesitate to jump into the thick of things to save another survivor. Their survivals are mutually dependent on each-other, and if he or the others failed to at least try and render assistance when it was needed by anyone else around them, then he would become the same selfishly driven person that he sees (and so, so despises) in the rest of the human race in general; something he would rather die than do without a damn good reason.
One dialogue snippet that would probably encapsulate him fairly well;
Other Survivor: "If you hate human beings so much, why are you helping us?"
Him: "If I didn't, then I'd be just like the people I hate. The world being a pile of crap is no reason to make it worse. Besides, you guys don't seem so bad."
Preferred weapons;
Assault Rifles or some shotguns.
Any side-arm with decent stopping-power, so he'd prefer a .44 or a .357. Not so much for .50s, since the recoil on those is usually too big to be worth it.