This one is a little easier. D— is blackmailing the royal lady, and using his power over her to get what he wants politically. He's also the enemy of Dupin, our protagonist. That alone makes him the antagonist. When we consider the fact that Dupin, G—, and the royal lady want to get the letter back from him so they can continue to deceive the king and cover up the royal lady's (presumed) affair, though, D— seems like just another corrupt player in a city of corrupt players.