King Arthur is the protagonist of The Once and Future King and the novel’s narrative and emotional center. The novel follows Arthur’s life from beginning to end, and the major events in his life shape the story. After Arthur becomes king, his ideas about government reshape English society, and these changes determine the plot, chronology, and setting of the four books that make up the novel. Even the novel’s title promises that although the story ends with Arthur’s death, he will always be England’s ruler. Despite Arthur’s extraordinary importance to the novel, however, he is a fairly simple character. As a child, Arthur (then called the Wart) is honest, trusting, modest, and good-hearted, and he preserves these qualities when he becomes king. King Arthur shapes his government with an important new philosophy that makes him a great king, but the ideas are Merlyn’s rather than Arthur’s. Arthur is exceptional because he believes in these ideas and is able to enact them when he becomes king.

Arthur develops a sense of world-weariness and wisdom in the novel’s later books, but this development is gradual and his basic nature is not drastically altered. Benevolent optimism keeps Arthur from acknowledging Lancelot and Guenever’s love affair early in the novel; later, the same benevolence causes him to persuade them to keep their behavior secret. Even as he grows older and wiser, Arthur is incapable of acting harshly toward the people he loves, no matter how hurtfully they treat him. In a sense, it is Arthur’s very simplicity and earnestness that enables the downfall of his reign. While the direct cause of the tragedy is Arthur’s incestuous affair with Morgause, we do get a sense that Camelot is also doomed because it has stagnated. The energy and progress of Arthur’s early reign slows to a halt, and Arthur becomes a defender of the status quo. This lack of innovation sets in around the time that Nimue imprisons Merlyn, suggesting that Arthur cannot think and develop without his old tutor. It is as though Arthur can only ride the momentum of his earlier ideas without forming any new ones. As Camelot stagnates and the quest for the Holy Grail takes its toll on the Knights of the Round Table, the Orkney faction is able to gain more power, until Camelot is too corrupt to survive.