Duke of Cornwall
Cornwall, Regan's husband, represents abuse of power at its worst. Lear is certainly one for getting angry and making poor decisions, but Cornwall's anger has a sadistic edge. He enjoys causing other people pain, and he likes being in power because then nobody is allowed to stop him.
You can tell Cornwall is mean and power-hungry when he puts Kent in the stocks for failing to show a little respect, but the real stuff comes out when plucks out Gloucester's eyes for giving King Lear shelter near his castle. Cornwall cold-bloodedly explains that he wants to hurt Gloucester just for the pleasure of it, and that nobody can stop him. "Though well we may not pass upon his life / Without the form of justice, yet our power / Shall do a courtesy to our wrath, which men / May blame but not control" Cornwall says (3.7.3). In other words, Cornwall knows he can't get away with literal murder, but he can still get away with murder. So this is the essence of Cornwall: "power" is just a tool to gratify sadistic tendencies.