The friend in every group of friends who no one really knows why "that" friend is there. "That" friend is usually the butt of every joke, prank, or the one who follows every request made by someone in the group of friends. Also, the "that" in "that" friend usually has a different tone than the rest of the words.
Rick: Hey, where is your friend?
James: Which friend?
James: Oh, "that" friend. We sent him out to go bring us some drinks.