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