Character Analysis
King Arthur rules all of Britain, and while he seems decent enough, Hank doesn't think much of his intelligence:
Well, I liked the king, and as king I respected him—respected the office; at least respected it as much as I was capable of respecting any unearned supremacy; but as men I looked down upon him and his nobles. (8.8)
Arthur embodies all of the prejudices and preconceptions of the age: he's entitled, he favors the nobility, and he doesn't see anything wrong with treating the rest of the country like his personal toilet bowl when he feels like it. We can see these qualities most clearly during his incognito adventures with Hank, particularly when Arthur comments on a pair of hanged bodies: "If others hanged him, belike they had the right—let him hang" (30.5). Oh good—this guy's in charge.
Luckily, Arthur can be changed… though it takes a lot more work on Hank's part than it does with Clarence. Hank gets Arthur to abolish slavery only after he's taken as a slave himself, and while he finds Hank's plan to go undercover as a peasant exciting, he almost blows the whole thing because he can't talk like anything but a king. It's hard to teach a fancy dog new tricks.
Even so, Arthur does grow and learn, and as Hank realizes early on, there's not much he can do against the Yankee's power and influence:
I was the substance; the king himself was the shadow. My power was colossal; and it was not a mere name. (8.2)
Hank's ability to sway Arthur, to shift his course of action, makes his death a larger part of Hank's ultimate tragedy. Since Arthur learns from his mistakes and improves as a person, he is another instance of Hank's methods working, thereby adding to the loss when Hank's imprint on the Middle Ages disappears.
Though powerful, Hank's not mighty enough to save Arthur from the clutches of classic Arthurian literature. The minute Hank turns his back, Arthur and Launcelot start fighting over Guenever, Arthur loses his kingdom to Mordred, and Mordred eventually kills him. Even with all of Hank's alterations, some things never change... and Arthur ends up dying the same way he did in all the other legends written about him.