I've only seen Mark at social gatherings, never in person. Parties, outings, conventions, group trips to the movies, that kind of thing. But he's only there retroactively. At the time, he's not there, only when you write about the event afterwards.
That's the other weird thing about Mark. He's only there retroactively when you're writing about the event, not when talking. When I'm talking over the phone, I haven't heard of him. But over Yahoo! conversations with friends, everyone knows him - but nobody knows him well.
Mark has always been friendly enough, but weird. He's only laughed at jokes that would still be funny afterwards. If "you had to be there," he has never laughed at it.
He has explained this to me a few times, and always seems comfortable with it. He knows he's different, ontologically, and he's okay with it. Online and at parties, people seem willing to deal with it, but I've never asked my friends, in person and one-to-one, what they think of Mark.
He looks nondescript, of course. Just imagine him looking like an inexceptional member of your own social culture.
What does Mark do for a living?