Petulant is best characterized by his name. Obviously, as young Witwoud is excessively good-natured, not even recognizing an insult, Petulant is ill-natured, too eager to prove himself by ill manners. He too, like young Witwoud, is a pretender to status. He is a liar, says young Witwoud, a poser, and, of course, petulant.
He is an interesting specimen in that he talks of "having a humour" to do something or other — the sure sign that he is affecting the humour, although it may by long use have come to be, by Congreve's distinction, a habit.