The Unchosen One: In the Prequel of Book VI, everyone but Sallustius is skeptical about the whole "making a low-grade of the militia governor of Britain". Too bad for them (and for Sallustius) that "Arturus" proves to be too good at the job.
Hot Guy, Ugly Wife : Him and Guenièvre; He's popular with women , and has many mistresses. Guenievre, on the other hand, as been called ugly on more than one occasion (Lancelot would disagree, however).
Mole in Charge: The initial plan of the Romans, which failed due to Arthur siding with the locals and playing both sides to allow Sallustius to keep up appearances, while effectively removing all Roman presence in Britain.
Informed Flaw: Léodagan, Guenièvre, Lancelot, Séli and even Arthur all agree several times that he is a "complete moron" ("especially with his wife!"); yet, on screen, not only is he a very polite and tolerant guy, but he is one of the few Knights of the Round Table who's both competent and reliable.
Flanderization: Starts in the show as... Karadoc (seriously, just look at his tropes), and comes out in Livre V as a somehow dumber version of himself. His first act as King of Britain? Do the introductions at the Round Table. He's been working with those guys for five bloody seasons, for crying out loud.
Idiot Savant: He is utterly clueless about map-reading and cardinal points, can't go in a forest without getting lost, don't understand a thing about dates and fail even the most basic logic... Yet he is a goddamn genius with numbers and mathematics (can do mental calculations lightning-fast, understands games with impossibly complicated rules, is apparently unbeatable at the shell game...).
Irrational Hatred: Mevanwi is the only person Perceval is downright mean to, even before he got to know her. Of course, he's proven right when she reveal much later how much of a Manipulative Bitch she is.
Arthur: You know Britain is an island, right? (...) Arthur: And when invaders want to raid the country, where do they come from? Yvain: They come from thousands of people? Arthur: ...Nope... Yvain: They come from... Britain!
Real Men Eat Meat: Throwing chunks of raw meat at him is the only way to break his murderous tendencies.
Guenièvre / Guinevere (Anne Girouard)
Queen of Britain.
Character Development: In Livre I, she was notorious for her rather clueless attitude and lines ("Ah yes, today I had the rosebush in the backyard clipped, because he really needed it!" to Arthur, while he's reading some diplomatic treaty); in Livre V, she is the only one who really manage to make Arthur understand that he is doing wrong, even saving his life at one point.
What the Hell, Hero?: delivers a mild one to Arthur about his suicide attempt because the sight of his blood all over the place still gives her nightmares six months or a year later, and she feels guilty because she was the one who prepared the bath.
Straight Edge / The Teetotaler: In the episode "Corpore Sano", he says himself that his feeding is made of "Dried fruits, beans, lentils, and above all, lots of water; and of course no alcohol." It is also implied that he doesn't eat meat. It is also worth noting that, in "Corpore Sano II", he's going through a nervous breakdown that makes him want to abandon his philosophy.
You Fight Like a Cow: His favourite way of fighting. According to him, it is necessary to motivate yourself to fight a friend.
Loth Just between you and me: what do you think of King Arthur? Dagonet: (hesitates) Loth: You can talk with sincerity; long story short, you're at scums' home here: our ideas aren't very nice, and we're not afraid to say it; we instigate, we renegade, we let our fantasies run wild.
Pragmatic Villainy: Despite being an evil, proud and treacherous king, he's smart enough to realize that his attempted coup will fail, and doesn't hesitate to sacrifice his pride by going before Arthur and apologize (well, sort of) when he realizes that this is the only behaviour that may prevent him from being executed (and it does).
Sesquipedalian Loquaciousness: He has a significantly larger vocabulary than most of the other characters, and he won't be afraid of using lots of metaphors.
Foil: to Arthur. By all accounts, Uther was bloodthirsty, short-tempered and archaic. Even though many characters who knew him (like Ygerne or Léodagan) like to berate Arthur for being too progressive and compromising for their tastes and how he doesn't stand the comparison, they occasionally admit that Arthur is probably an easier king to deal with.
Undying Loyalty: Played With. He's arguably the most selfish character in the show, is in here only for his own profit, going as far as organizing a burglary in Kaamelott. And yet, he says several times in the show that things are better when Arthur is king, and he is the one, in Livre VI, who help Arthur to hide and flee to Rome when Lancelot and his army starts chasing knights around the kingdom.
Rivals Team Up: When he's forced to work with Merlin, to his disappointment.
Sociopathic Hero: "In life, you must avoid working for someone, AND avoid making someone work for you; it's the key to success."
Superpower Lottery: Even though he hasn't really got "one ungodly over-the-top superpower", the variety of magic he can use still makes him qualify for the trope. During the series, he is indeed either seen or said to master: destructive spells (such as lightning, fireball, stone rain, disintegrate, and the notorious "Death Blow": "he blows on you, you DIE!"), teleporting, demonology, necromancy, shapeshift (into another human being, at least), "beastmastery" ("Caller of the Caledonian Wolves"), enchantments, divination, alchemy... and maybe more.
"All of this is nothing. Absolutely nothing! Not long ago something did "boom". There, we're on a tiny crumb expelled by this boom, drifting and turning on itself, waiting for its end; near! On this crumb little organisms gesticulate; born, alive, dying. None of this has any consequence."
Shout-Out: A machiavellian "evil god", wearing a long black mantle with a large hood, trying to "corrupt" Lancelot and Arthur (a.k.a. the most willing to do good) and to destroy Logres, the only kingdom trying to find the Grail? Hmm.
Merlin (Jacques Chambon)
Butt Monkey: While a lot of character could fit, Merlin takes the cake by being the guy who takes the most slaps, kicks, punches or "headbashes-to-the-table" on-screen, mostly from Arthur and Elias. This, plus the various dirty tricks from Elias.
Arthur: Is it true that you're the son of a demon and a virgin? [...] You've taken more from the virgin.
Inept Mage: Possibly subverted; Merlin himself claims to be quite effective, but as a Druid. Of course he's useless in a royal court.
Unreliable Narrator, really: everytime he talks about his druidic powers, it is implied that he really is inept; for instance: when, in Livre VI, in the streets of Rome, Arthur asks him if he could strike the place they're in with a lightning bolt (something Merlin explicitly said he could do, as it is a druid's specialty), Merlin replied that it would be a lot easier to do if there was a storm going on.
Involuntary Shapeshifter: Played with. He can control when he transforms, but not what he'll turn into (It depends on whatever spirit is celebrated that week).
Man Child: While he is shown to have many good sides (deeply in love with his girlfriend, the closest thing Arthur has to a loyal friend in the whole saga, etc.), he seems unable to deal with a difficult situation without being an immature brat.