Depending on the Writer: Who appears,who has major and minor roles, who is related to whom, how characters act... and pretty much everything else.
Odd Name Out: King Arthur's nephews are Gawain, Gaheris, Gareth, Agravaine, and Mordred. Guess which ones are evil.
One Steve Limit: Averted. There are several women named Elaine, and there are several different women who occassionally share the title "Lady of the Lake". There are also three Iseults - Tristram's lover (Iseult "the Fair"), her mother, and another princess who Tristram marries (Iseult "of the White Hands"). There are also two Ectors, Arthur's foster-father Ector and Lancelot's half-brother Ector de Maris, and two men named Bors, King Bors and his son Bors of the Round Table.
Spell My Name with an S: Variant spellings of names abound. Some distinct characters may have even resulted from people interpreting spelling variations as different people (ex. Gaheriet and Guerrehet = Gaheris and Gareth; Morgause and Morgan). A manuscript of a work may even have different spellings for a name, which can then differ with other manuscripts of that same work (ex. Guinevere).
Characterization Marches On: Although he was always a warrior hero, early traditions depict him as less of a clean-cut good guy than later interpretations, being quite lustful, jealous, prideful and greedy. He quarrels with churchmen, tries to steal Tristan's pigs, kills a rival over a woman, and fathers several sons, none of whom is linked to his wife Guinevere.
Depending on the Writer: Arthur is pretty much the gauge by which you can read the Arthor's opinion on proper kingship. Thus, in the Welsh legends he does his own Asskicking Equals Authority and leads from the front and challenges the church on occasion, while to Mallory and the French he's your typical wellmannered and cuckolded King who leads from behind and isn't actually that great of a fighter. Modern writers have made him badass, cowardly, conflicted, compassionate, and tyrannical. And usually they do that while telling the exact same plot!
The original Arthur may not have even been a king himself, since he is called dux bellorum or "leader of battles", who fought "together with the kings of the Britons" in twelve great battles against the Anglo-Saxons. He is also called "Arthur the Soldier" in early material. In later works he is called the High King of all Britain and even Emperor, and he even conquers Rome.
Nice Job Breaking It, Herod!: Arthur decides to round up all of the babies born on May Day and send them out on a rickety boat because Merlin prophesied that a child born on this day would destroy him. One baby (Mordred) survived.
Out of Focus: In the literature, his knights like Lancelot get more and more of the spotlight and he is almost if not actually Demoted to Extra until the story covers his downfall.
To Be Lawful or Good: Modern writers tend to make him quite conflicted over his decision to burn Guinevere at the stake in order to show that the queen isn't above the law, with some even depicting him as secretly hoping Lancelot will save her. Originally, this was very much not the case.
Composite Character: Originally based on the wandering mystic Myrddin Wyllt, he became merged with Romano-British warlord Ambrosius Aurelianus by Geoffrey of Monmouth. Remnants of this remain today with some modern fiction referring to him as "Merlin Ambrosius."
Omniscient Morality License: Merlin gives Uther the appearance of Gorlois so he can father Arthur upon Igraine, then has Arthur snatched away at birth and given to Ector, arranging the "sword in the stone" test as well, presumably so that events would happen as he prophesied. Not revealing Arthur's true parentage led to many rebellions during the early years of his reign, as well as the conception of Mordred and the May Day massacre.
Chickification:Perhaps the Ur Example. While in most Arthurian stories from the 13th century onwards she's not action-oriented at all, in the oldest Welsh tales she was a badass warrior and magic-user. Only very recently have we seen a swing of the pendulum back in favor of more Action Girl-esque portrayals.
Evil Identical Half-Sister: On the same night Leodegrance fathered Guinevere, he also begat another girl on the wife of his seneschal. This "False Guinevere" was born on the same day, looked exactly like her half-sister and even had the same name (Leodegrance had strong genes, but it looks like he wasn't very imaginative). She managed to trick Arthur into thinking the was the real deal, and tried to have Guinevere mutilated and banished (with Lancelot's help, she escaped). "False Guinevere" kept the charade up for years until the Pope himself stepped in and demanded Arthur take the real Guinevere back. He refused, and the Pope interdicted Britain for twenty-one months. After ten, "False Guinevere" had a stroke, lost every sense but sight and hearing, then began to rot alive, until she finally confessed and perished.
Spell My Name with an S: Guenevere, Guenever, etc. The Welsh form is Gwenhwyfar. (A modern equivalent is Jennifer.)
In just one manuscript of Geoffrey of Monmouth's History of the Kings of Britain, Guinevere's name is spelled differently every time she's mentioned (Ganhumara, Ganhumera, Guenhumara, Guenhuuara). Other manuscripts have more variations.
Executive Meddling: Lancelot's first appearnace is notable for being unfinished by Chretian De Troyes. Some think this is because his patron, one of Eleanor of Aquitaine's daughters, may have ordered him to add the infamous affair with Guinevere. In other words, it's possible the entire crux of a huge portion of the Arthurian Romances was the result of a lady wanting to turn an adventure story into the medieval equivalent of a Harlequin Romance novel.
Fatal Flaw: His illicit affair with Guinevere proves to be the undoing of the Round Table itself.
Brave Scot: Usually associated with Lothian and/or the Orkney islands, both part of Scotland.
Chick Magnet: In both Mallory's take and in Gawain and the Green Knight, he is glomped on by a female paramour with him doing nothing to attract their attention. With the lady of the Green Knight, he refutes her advances, because it wouldn't be honorable.
Chivalrous Pervert: He's definitely willing to reciprocate other advances though. And he's even willing to honor deals with less beautiful women like Ragnell, which ends up getting her being smoking hot and him being a proto-feminist.
Clear My Name: One of his adventures in Parzival has him doing this.
Indy Ploy: One episode in Chrétien de Troyes's Perceval and Wolfram von Eschenbach's Parzival has him holding off an angry horde of townspeople while using a chess board as a shield.
Of course, the chess pieces are described as being ten times as large as normal ones, and are also used as impromptu missile weapons at one point, so (assuming a board of equivalent size) that's actually not too impractical.
The Lancer: Earlier stories suggest he was originally Arthur's second in command and one of the successful heroes of the Grail quest. Adds a new light to his rivalry with Lancelot, doesn't it?
The Power of the Sun: Some versions of the legends have him gain strength as the sun rises in the sky and lose it when it sets, whether as a function of his magic sword or of his being an Orkney.
Celibate Hero: In some versions, and in most versions of the Grail Quest his hardest test is resisting a beautiful enchantress.
Cool Sword / Blessed with Suck: In one set of stories gets a sword that can cut through anything and will never break... except in the toughest battle of his life.
David Versus Goliath: Pretty much literally in some versions. When he first comes to Camelot, he's just a young guy armed with some javelins and no armor to speak of and takes down a huge and powerful knight that the knights of Camelot were afraid to fight.
Heterosexual Life-Partners: In some stories with Gawain, who is his cousin. In one legend he even chooses to share a curse Gawain brought upon himself; his willingness to sacrifice saves Gawain's life by splitting the curse in half and they each only get badly wounded.
Idiot Hero: In most stories, he's this in his younger days. Sometimes he grows out of it and into The Messiah, sometimes he doesn't.
Undead Author: He serves to avert this problem. After the Grail quest is complete, Galahad ascended to heaven and Percival went off to be a monk. Bors was the only one to return to Arthur's court to tell the tale.
Magic Knight: In his earliest incarnation from the Welsh tales.
Never Live It Down: In-Universe. When Percival first showed up to Camelot, a woman who had previously sworn never to laugh unless she saw the man who would be the best knight in the world. When Percival arrives, she laughs. Kay is so insulted that he slaps her. Throughout the rest of the story, Percival's defeated foes keep coming back to Camelot to tell of how Percival was still trying to avenge her honor (basically to rub in how much of a douche Kay was).
The Nicknamer: An example being Sir Gareth, who he dubs 'Beaumains' after the former refuses to reveal his name.
Playing with Fire: In the Welsh tales Kay can generate so much body heat that he can keep dry in rain.
The Worf Effect: He seems to have a habit of getting the crap beat out of him to show how much better the new knight du jour is.
Bash Brothers: Often paired with Kay in the oldest Welsh material.
Battle Butler: Arthur's cup-bearer in later versions. Sometimes has a brother, Lucan, who is Arthur's designated butler.
Composite Character / Expy: In several modern Arthurian retellings favoring the older traditions as opposed to Malory et al, Lancelot does not appear. Instead, Bedivere is often made Guinevere's lover. It helps that many of Lancelot's usual traits apply to Bedivere also. This was first done in Rosemary Sutcliff's Sword at Sunset (1963) and a number of other works have followed suit.
Demoted to Extra: Just like Kay, his role was much reduced as the mythos grew and developed.
Child by Rape: Medieval versions of the tale have Morgause using magic to get Arthur to sleep with her. Some modern versions have Arthur raping her.
Evil Uncle: Inverted. In the early versions of the story, Arthur is his uncle and he's the Evil Nephew.
Fallen Hero: Both in the literal and meta sense. He's usually depicted as at least competent and loyal to Arthur until he finds out who his father is. However, the original legends take this even further, implying he's Arthur's peer and ally, biologically his nephew and legally his foster son. The original reason for their inevitable confrontation is supposedly tied into a spat between Gwenhwyfar and her sister Gwenhwyfach.
Face-Heel Turn: Actually seemed quite promising during the first two years of his knighthood, earning praise from Lancelot himself.
Fake King: Yes and no. Arthur leaves him in charge of the kingdom when he goes to fight Lancelot in France, 8and he does officially have himself declared king, but he isn't the rightful ruler.
Heroic BSOD: Being told that you're the bastard son of the king and his half-sister, prophesied to destroy the kingdom and do more harm in your lifetime than all your ancestors did good kind of does that to you. He came out of it...uh, pretty badly, actually.
Humans Are White: Not the only ex-Saracen knight, but certainly the most prominent.
Kick the Dog: On the receiving end of one. After she convinces Tristram to spare his life following on of their clashes, Isolde makes Palomides deliver a message to Guinevere stating that there were but four true lovers within the land: Guinevere and Lancelot and Tristram and Isolde. Ouch.
Your Cheating Heart: Not his, but his wife's. She hid his clothes so she could run off with her lover. Wolf-Marrok tore off her nose, and King Arthur tortured a confession out of her. She then got exiled with the lover and had a bunch of noseless kids.
The Smart Guy: Possessed a great deal of common sense, and is one of the few knights who can identify other knights by their faces as well as their coats of arms. Given the sheer number of duels caused by mistaken identities in these stories, this makes him pretty useful to have around.
Blood Knight: Settled down for a bit after marrying Laudine, but Gawain showed up and tempted him back into adventure. Laudine agreed to let him go Walking the Earth, but only if he would return to her after one year. She eventually had to send someone to hunt Ywain down and tell him not to bother coming back.
Disproportionate Retribution: Esclados the Red, guardian of a supernatural storm-causing fountain, (non-fatally) fought Ywain's cousin Calogrenant after he came upon said fountain and caused a dangerous tempest. Ywain, upon hearing of this, kills him and marries his widow.
Historical Villain Upgrade: Modern writers tend to make her a scheming villainess, often by making her aware of her relationship with Arthur when she sleeps with him, or by combining her with Morgan Le Fay. Originally, she seems to have been a generous and friendly person. Very friendly.
Honey Trap: Malory states that Lot sent her over to Arthur's court in this capacity.
Out Shortly After A Bang: Her son Gaheris, incensed at seeing her in bed with the son of Pellinore (who killed Lot, the Orkney brothers' father), lopped her head off.
Green-Eyed Monster: Apart from the incident in Your Cheating Heart below, part of Morgan's hatred for Guinevere may have stemmed from the fact that she herself wanted to sleep with Lancelot.
Heel-Face Turn: In time she seems to have mellowed out and gotten along well with Arthur, to the point of letting him visit her castle for a week. Of course she still tried to convince him of Guinevere's unfaithfulness, but he didn't believe her.
Death by Origin Story: Arthur becomes king because of his death, being his son and all, but he is plucked from a life of obscurity with a foster-family and thrust onto the throne at a young age. Details vary though.
Names to Run Away From Really Fast / Awesome Mccoolname: Pendragon is a Welsh epithet (sometimes treated as a surname or dynastic name by later authors) which translates to "Chief Dragon" in English, "dragon" probably being a figurative term for warrior. "Uther" itself may come from a Welsh word meaning "awesome" and/or "horrible/terrible" (in the original sense of "fearsome" rather than "bad").
King Lot of Lothian and/or Orkney
Adaptational Villainy: Wasn't a villain in Geoffrey of Monmouth, but is one from the Vulgate Cycle onward.
Even Evil Has Loved Ones: Father of Sir Gawain and his brothers, who in some versions pursue a Blood Feud with King Pellinore and his family after Pellinore kills him - despite Lot having rebelled against Arthur.
Starter Villain: The leader of a group of rebel kings in the early years of Arthur's reign.
Black Knight: Sometimes fights one. Sometimes is one, as in Howard Pyle's retellings.
Defeat Means Friendship: He defeats Arthur after being challenged to joust. The Sword in the Stone breaks during their fight, to be replaced by Excalibur. Arthur later asks him to join the Round Table.
Meaningful Name: He is completely green, as well as his gear and horse, and he carries a bough of holly in one hand (and an axe in the other). Some scholars think he may have pagan roots as a naturefigure.