For someone who's the title character, King Arthur of England sure doesn't get too much face time in Le Morte D'Arthur, especially in comparison with star knights like Launcelot and Trystram. That said, we do know a fair amount of things about everyone's favorite king.
For starters, he's really awesome. He's pretty much the best king ever, in fact. Or at least according to his supporters. As his knights travel around on adventures, they're always telling their opponents that they serve the greatest king who's ever lived, and the narrator constantly refers to Arthur as the "most noble" king.
If that doesn't sell you, then consider the fact that Arthur manages to command exceptional loyalty from his knights – so much so, in fact, that even when Arthur attacks Launcelot's castle, Launcelot declares himself unwilling to do battle with the king who made him a knight. Arthur wouldn't command all this loyalty and respect if he weren't doing something right. But what is it that he does so well?
It's certainly not that he's a great fighter. True, he leads his troops into battle without hesitation. And yes, the guy is totally brave and chivalrous – so much so that he takes time out of his battle march to Rome to rescue the people of Bretagne from a dangerous warlock who's been eating children and raping maidens (thanks, Artie!).
But as the story progresses, Arthur spends less and less time fighting and more time as an organizer and spectator of jousts (through which other knights prove their prowess), and a receiver of hostages captured by the other, more active knights. Rather than being a fighter, then, Arthur's role is to serve as a center of power and judgment, a dispenser of rewards and titles to those who prove themselves on the battlefield.
In this, Arthur proves himself to be an exceptionally great feudal lord. Feudalism, a system in which a lord protects the men who swear loyalty to him (his vassals), in exchange for these vassals' agreement to go to battle for that lord, depends upon a give-and-take of promises. For vassals to have confidence in their lord, they have to know that he'll keep his word. After all, what's the point of showing loyalty if you're not absolutely positive it will be rewarded?
Lucky for his vassals, Arthur is really good at keeping his word, perhaps more than any other character in the story. In "The Tale of Sir Gareth," Arthur even grants young Gareth the quest of a damsel-in-distress, despite the fact that the knight is un-tested and a failure on his part would spell trouble for Arthur's precious reputation.
Why does he bother to give the young kid a chance? Frankly, it's because he has promised to grant Gareth whatever he requests. His failure to do this would be a worse failure than anything the un-tried kid could do. It would mean Arthur's dishonorable, and these guys are all about honor. So in the end he'd rather risk the kid's failure than seem like a promise-breaker to his subjects.
The full strength of Arthur's word becomes all the more clear when you compare him to King Mark, who's pretty darn devious. For one thing, he fails to keep his promise to Arthur to treat his nephew and vassal Sir Trystram with honor. As Sir Bors says, "ye know well that Kynge Arthure and Kynge Marke were never lyke of conducions, for there was never yet man that ever coude preve Kynge Arthure untrew of his promyse" (654.22-23). So the shadier this Mark guy seems, the more honorable and trustworthy Arthur seems in comparison. This, more than anything else, is what enables Arthur to command such loyalty and respect from his knights. They see how much worse they could have it, and they're grateful to serve a respectable guy.
Is Arthur a perfect king, then? Well, not exactly. See, Arthur may be a great feudal lord, but he's also human, with very human imperfections. One of these imperfections is an unwillingness to deal with things that make him a bit uncomfortable. Hey, we've all be there. Denial ain't just a river in Egypt, right? Right?
Despite receiving several clues that Launcelot and Gwenyvere are having an affair, Arthur chooses to ignore it, either because it's too painful, too awkward, or he just doesn't want to know. Even when forced to see the writing on the wall, he opts to let Mordred and Aggravayne deal with the problem rather than face it himself. You could even argue that this failure on Arthur's part leads to the downfall of the whole Round Table, because the rift created in the court by Gwenyvere and Launcelot's affair brings the knights to war. It appears our Arthur is not above making some big fat mistakes.
Plus, there's the rather indelicate matter of the fact that Arthur has sex with his own sister, and sires a son, named Mordred. Oops doesn't even begin to cover that one. This youthful, shall we say, indiscretion also contributes to his downfall in the end. But hey, wait, Arthur doesn't know it's his sister. Let's go a little easy on the guy, you might say.
But should he have been having sex with a strange woman in the first place? Probably not (although if he's basing his behavior on all the other men around him, then he doesn't exactly have a ton of great role models). And again, rather than deal with the problem in his own way, Arthur chooses to follow Merlin's advice to kill all the baby boys born on May Day that year – a very unkingly action for which, it could be argued, Arthur's eventual death at Mordred's hands is just desserts.
Our point here is that, like everyone, Arthur really bungles it now and then. And since he's the king, his mistakes have consequences not just for him, but for his whole kingdom, too.