If you are referring his defeat at the end against the dragon, that is because of his hubris. He was too proud and believed that he could do it alone without his soldiers. This brings his defeat and he realizes that he will die because of his hubris, after which Wyglaf will take over since he was the only one brave enough to help.
Surprisingly, his hands and feet are bigger than normal, which helps him move quickly through the water.
The King and Queen are the antagonist. If you are looking for that what exactly is the question?
