There are so many minor characters mentioned in Le Morte, that it just might take days to name them all. But we'll go ahead and give you the lowdown on the ones worth remembering.
Accalon is Morgan's man. He's involved in her plot to take down Arthur, because he hopes that by marrying Morgan and killing Arthur, he will be crowned king. The worst part about this is that Accalon is one of Arthur's knights, who goes on many a heroic adventure. Maybe that's why Arthur forgives him for being a part of Morgan's nefarious plans. Unfortunately, though, that forgiveness comes too late and Accolon dies of wounds he received a fight with Arthur.
Balyn and Balan
These two brothers with annoyingly similar names only pop up in their own tale. Balyn, embroiled in some drama in the court, disgraces Arthur when he kills the Lady of the Lake. In an attempt to get back into Arthur's good graces, Balyn accumulates a massive body count worthy of Jack Bauer, which culminates when he and his brother Balan accidentally kill each other because of a case of mistaken identities. Whoops.
Ban and Bors
These two brothers from Gaul (France) come to Arthur's aid in his war with the alliance of Northern Kings. This alliance gives Arthur some lands in France, which he ends up losing in his tiff with Launcelot who is, as it turns out, Ban's son. Bors, by the way, goes on the Grail quest, and is one of three knights who succeeds in seeing the Holy Grail, because he's a pure and devout Christian.
This is the guy that raised Arthur from infancy, after he was conceived by King Uther and Igrayne. After Arthur pulls the sword from the stone and becomes king, Ector joins his court. He later goes on the Grail quest, but fails to see the Holy Grail, probably because he wasn't quite as worthy as Galahad, Percyvale, or Bors.
The Two Elaynes
Launcelot has affairs with two women other than Gwenyvere in the course of Le Morte, and both of them are named Elayne. The first, Elayne of Corbin, is the daughter of King Pelles and conceives a son with Launcelot, Galahad, who goes on to fulfill the Grail Quest. The second, Elayne of Ascolat, meets Launcelot when he rides incognito to a tournament and nurses him back to health after he receives terrible injuries there. She dies of grief when Launcelot refuses to marry her, and has her father send her body to Camelot in a barge, her lifeless hand clutching a letter telling the story of her love and death.
Both of these Elaynes play essentially the same role in Le Morte: they reveal the utter selfishness of the affair between Launcelot and Gwenyvere. Beyond the obvious treachery involved in adultery, Launcelot and Gwenyvere's affair prevents Launcelot from marrying women who have obvious claims on him; in Elayne of Corbin's case, because she's the mother of his son, and in Elayne of Ascolat's because she is perfectly suited to be his wife and is utterly devoted to him. She nursed him back to health from the brink of death, for crying out loud. Yet Launcelot and Gwenyvere's affair prevents these relationships from turning out the way they should, just as it eventually disrupts the feudal relationships of the Round Table.
Gaheris and Aggravayne
These two rather unsavory characters are Morgause's sons (brothers of Gawain and Gareth). They scheme to reveal Gwenyvere and Launcelot's affair, and Gaheris is later killed by Launcelot when he rescues his beloved Gwenyvere from execution. That'll teach him not to interfere with true love.
Oh, and these two, along with their brother Mordred, are also a part of the rather unknightly killing of Lamerok, who is the son of their father's murderer.
Wife of the duke of Cornwall, Igrayne immediate captures King Uther's heart. This prompts him to orchestrate a rather shady deception in which he disguises himself as her husband, sleeps with her and then, lo and behold, nine months later, Arthur is born.
Kay is Ector's son, and therefore he's Arthur's foster brother of sorts. When Arthur becomes king, Kay becomes an early member of Arthur's Round Table. He doesn't do as much as many of the other nights, but he does have a habit of giving other knights revealing nicknames like Beaumains and La Cote Male Taylé.
Lady of the Lake
This mysterious woman gives Arthur a new sword in exchange for a favor she'll cash in later. That favor turns out to be the head of Balyn, who killed her brother. This prompts Balyn to kill her, and there goes the Lady of the Lake.
This guy is one of those well-loved knights of the Round Table. There's just one problem: his father killed King Lot, who is Gawain, Gaheris, Aggravayne, and Mordred's dad. So Lamerok dies an unfortunate and gruesome death at the hands of these knights, who stab him in the back.
Morgause's husband, and father to Gawain, Gaheris, Aggravayne, Gareth, and Mordred, King Lot dies when Pellynore bests him. To be honest, we're not fans of Lot from the get-go, because he starts a war with Arthur over the throne. This, coupled with his death at the hands of Pellynore, starts a serious blood feud that helps bring an end to the glory of Camelot.
The King of Cornwall, Mark is Isode's husband, and therefore Trystram's rival in love. But he's also Trystram's overlord, which means that he demands absolute loyalty from the very knight who's sleeping with his wife. Mark is less forgiving, and far less honorable than Arthur, and his unknightly behavior toward Trystram and the other knights makes him the king we all love to hate.
Morgause is Arthur's sister and King Lot's wife. Which is all well and good until Arthur sleeps with her and she gives birth to Arthur's illegitimate son Mordred. As King Lot's wife, Morgause already dislikes Arthur, who is fending off attempts by her husband to take control of the throne.
Later she has an affair with, strangely enough, Lamerok. Remember him? The son of Pellynore, who killed Morgause's husband, King Lot? No wonder Gaheris is so upset to discover them in bed together that he decapitates his own mother.
This guy has got a serious crush on Gwenyvere. So serious, in fact, that he kidnaps her. Which prompts a whole kerfluffle with her knight-in-shining-armor, Launcelot. No surprise here, Launcelot kills Mellyagaunce at the end of this whole affair.
Pellynore brings this mysterious lady around. She's so alluring that Merlin falls head over heels for her. Unfortunately, she's just not that into him, so in order to put a stop to his advances, she buries him alive in a cave. And that's the last we see of Merlin.
If you remember one thing about this knight, it should be that he loves Isode. So much so in fact that he becomes a frequent opponent of Trystram in jousts. He's a pretty likeable guy, in the end though, and becomes buddies with Trystram (despite their romantic rivalry) and Launcelot. It's also worth noting that he starts off a pagan, but by the end of the story, he has become a Christian convert.
We first meet this man when he's hunting the questing beast and runs into Arthur along the way. While they get off to a rather rocky start, Pellynore eventually becomes a part of Arthur's crowd. His main contribution to the story is the fact that he kills King Lot in battle. This sparks a blood feud between their two families, which results in the sneaky murder of his son, Lamerok, at the hands of King Lot's sons.
Yet another member of Arthur's Round Table, Percyvale's significance doesn't become clear until the Grail Quest, when, because he's a good Christian and a virgin, he becomes one of only three knights who succeeds in seeing the Grail.
It all starts with Uther, folks. He's Arthur's proud pappa. Although now that we think of it, he can't have been too proud of the fact that the only reason Arthur is born in the first place is because Uther schemed his way into sleeping with an unsuspecting married woman. Maybe that's why he has Arthur sent away for his childhood. In any case, when Uther dies, he names Arthur as his heir, which starts the whole ball rolling.