One thing's for sure: Le Morte D'Arthur is action-packed. Characters may say one thing, but it's what they do that we (and the other characters) judge. Take King Mark, for example: he promises to be a good uncle and king to Trystram, and not to try to kill him anymore. Sure, that sounds great, but nobody believes him because that's not how he has acted in the past. Lo and behold, no sooner is Trystram back in Cornwall than Mark's back to his old tricks, sending Trystram to a tournament in the hopes of killing him. Really, Mark, must you be so predictable?
This concern with action may actually grow out of a concern with words – with whether or not a character is true to his word. Being true to his word was probably the single most important quality in a king or a knight, and the only way to know if someone is, is to make sure his actions match up with what he says.
So then the actions of most of the characters in Le Morte can basically be divided into two groups: knightly and not knightly. Yep, it's really that simple:
Mellyagaunce kidnaps the Queen? Definitely not knightly.
Launcelot rescues her? Knightly to a tee.
Trystram comes to Mark's aid as his champion in the dispute with Ireland? Totally knightly.
Mark tries to kill him? Totally not.
In all of these cases, the honorable actions are the ones with which characters fulfill their knightly roles; Launcelot is Arthur's knight and Gwenyvere's lover, so he owes it to both of them to rescue the Queen. Trystram is Mark's nephew and knight, so it's his job to serve as Mark's champion, whatever his personal feelings are about Mark. Acting in accordance with their roles is how characters earn the title of honorable or not, which, in this feudal world where chivalry reigns supreme, is pretty much all we need to know about them.
Many of the characters in Le Morte D'Arthur have descriptive names, sometimes given to them by another character. When Sir Gareth arrives at Arthur's court and refuses to give his name, so Kay gives him the nickname "Bewmaynes," or Beaumains, a French word that means "beautiful hands," because of his fair white hands that look like they've never done a day's work. Gareth's identity is in question at this point, but the nickname Kay gives him provides a clue: someone with hands like that is probably not just some hard-working commoner; we assume "Bewmaynes" is really a noble.
Kay nicknames another knight "La cote male taylé," which means "the poorly-tailored coat," because of the ill-fitting coat he wears. This coat actually belonged to his departed father who was killed by an enemy of his while he slept. The young knight plans to wear it until he takes vengeance on his father's killer, and this family history becomes incorporated into his identity with his nickname.
Other descriptive names tell us more basic information about a character. Isode is often called "La Beale Isode," or "the Beautiful Isode," because she's "the fayrest lady and maydyn of the worlde" (238.5). Pretty straightforward. Sir Breus sans Pité, which means "Sir Breus the merciless," is the kind of guy who runs around killing helpless maidens, and his name hints at that quality right off the bat.
Other character names are even less deep in meaning. "Sir Uwayne le Fyze de Roy Ureyne" simply means "Uwayne son of King Urien." We're not exactly thrown for a loop on that one. But even this kind of nametag can be important, because it lets us know what family a knight is from, an affiliation that's likely to have important effects on his interaction with other knights and his character, because knights tend to act according to family-inherited traits.