A person who will engage you in debate over the smallest matter of personal or professional decorum, preference, grammar, Et cetera (yeah you like that, I see you out there) in an attempt to satiate some kind of inner soul-oriented black hole (logic and reason come out on the other side).
We spent the better part of the ride on the subway exchanging the finer points of the efficient allocation of public space, where to stand, put your bag, and at what point you must move to the inside of the bench seats to let others sit down. Then he convinced some random girl to give up her religion. Then he got a black dude to say he'd vote republican in the next election. That dude is a masterdebater.