Sir Kay is based on the character of the same name from the King Arthur legends, who like the Disney character, was Arthur's adoptive brother. He is a big, muscular young man with red hair, and is most likely somewhere around the age of twenty, judging from a line from Merlin.
Sir Kay is the son of Sir Ector who, and at the beginning of the film, is in training to become a knight. Kay serves as a foil to Arthur, being self-centered, boorish, and lazy, despite being higher in rank than Arthur. Kay treats Arthur rather cruelly, usually calling him the deriding nickname of the Wart. However, at the end when the Wart becomes King, he comes to respect the Wart as the King and his foster brother. He also shows guilt for the way he treated the Wart in the past.
Sir Kay is first seen hunting deer with his bow and arrow. He aims at a deer, but misses due to the Wart. Kay sullenly returns to the castle, while the Wart travels into the forest to find Kay's missing arrow. Back at the castle, Sir Ector scolds Kay, who is eating a bowl of chicken legs, for allowing the Wart to go alone into the forest. Kay shows little concern, stating that he is not the Wart's keeper.
When Merlin appears, Kay notes that they should just run Merlin off, but Sir Ector shoots down this idea, fearing that Merlin would curse them if angered. A bit later, Sir Pellinore, a friend of Sir Ector's, comes with news of a tournament on New Years Day. Kay at first pays little attention, until Pellinore reveals that the winner will be King of England. Sir Ector notes that if Kay buckles down, he could be knighted in time for the tournament, and compete to win. Kay agrees, but not before Pellinore quietly notes that Kay as the new King is a dreadful thought. The next day, Kay is seen practicing his jousting, but it quickly becomes apparent that he is not skilled. Kay's next appearance is with Sir Ector who had been summoned to the kitchen. Having found the kitchen to be enchanted, he attempts to fight a mop, but is quickly defeated. He is seen to be fairly pleased when Sir Ector forbids the Wart from becoming Kay's squire.
At Christmas, Kay has apparently improved enough to be knighted. When his squire Hobbs comes down with the mumps, the Wart is called to be the replacement. Kay is shown to be disappointed, outright stating he does not want the Wart as a squire. There is no other choice, and so the Wart accompanies Kay to the tournament.
Sir Kay is waiting to fight when he discovers that the Wart has forgotten his sword. Kay threatens the Wart to bring it or not come back. When the Wart brings the legendary Sword in the Stone as a replacement, he is quick to realize it is not his, but is apparently ignorant as to what it really is. When the sword is replaced, to allow the Wart to prove he pulled it, Kay shoves him aside, and tries to pull it himself. Though Kay believes that it can be pulled after someone else pulled it, he fails. He is shown to be shocked when the Wart pulls it effortlessly, and he has to be reprimanded by Sir Ector, who tells Kay to bow down to his new King.
Knowing that Merlin prophesied that Arthur shall pull out Excalibur from the stone, Sir Kay arrives their first, wanting to become the Ruler of Camelot. Arthur warns him not to take it, but Kay defies him and does so anyway. As he wasn't meant to be the one to pull it out, the sword glows and turns Kay to dust.
- The way Sir Ector accidentally bumps his sword into Sir Kay's head and Sir Kay moans in pain when trying to attack the enchanted kitchen supplies is similar to the way Horace accidentally bumped his club/chair leg into Jasper's head and Jasper moaned in pain when trying to attack the puppies in 101 Dalmatians, only Sir Ector doesn't get kicked in the rear by Sir Kay for his clumsiness as Jasper does to Horace because he is Kay's father and he'll get mad at him for it.
- The main character for Don Bluth's video game Dragon's Lair, Dirk Daring, appears to have been modeled after him.