Sir Kay

In the Welsh Culhwch and Olwen, Cai is formidable: he can go nine nights without sleep, hold his breath nine days underwater, make himself as tall as a tree, and no physician can heal a wound from his sword. He is one of the great warriors of Arthur's band.

By the time French romance is done with him he is the seneschal — the household steward — and the court's designated bad-tempered fool: quick with an insult, quick to volunteer, and reliably unhorsed within a page. Every romance needs someone to be wrong first so the hero can be right, and Kay drew the job.

Malory keeps the French Kay but remembers the older one at the edges. He is the reason the sword comes out of the stone at all: he forgets his own sword at the tournament, sends his young foster-brother back for it, and briefly claims the one the boy returns with. And he is loyal to the end, dying with Arthur's household.

Malory keeps him useful in a specific way. Kay is the court's voice of ordinary unkindness — the one who names the unknown youth Beaumains and puts him in the kitchen for a year, who assumes the worst, and who is reliably shown to be wrong. Every romance needs a character whose judgement the reader is invited to distrust, so that the hero's quality can be established by contrast, and Kay does that work for three centuries without ever quite becoming a villain.

And he is loyal. He does not betray anybody, he does not go over to Mordred, and he dies with the household. The seneschal who forgot his sword at a tournament and thereby made a king is still at his post at the end, which is more than can be said for most of the men the book admires more.