Sir Kay was the son of Ector (Ectorious) and the foster brother of King Arthur. History records Kay (Cai in Welsh) as being a very tall man, as shown by his epithet, the Tall. He appears in the Mabinogion tale of "Culhwch and Olwen" as the foremost warrior at the Court of the King Arthur, and apparently had mystical powers and was called one of the "Three Enchanter Knights of Britain" for:
"nine nights and nine days his breath lasted under water, nine nights and nine days would he be without sleep. A wound from Cai's sword no physician might heal. When it pleased him, he would be as tall as the tallest tree in the forest. When the rain was heaviest, whatever he held in his hand would be dry for a handbreadth before and behind, because of the greatness of his heat, and, when his companions were coldest, he would be as fuel for them to light a fire".
Sir Kay at times had a volatile and cruel nature, but he was Arthur's senechal and one of his most faithful companions. Kay married Andrivete, daughter of King Cador of Northumberland, and he is credited with sons called Garanwyn and Gronosis and a daughter called Kelemon. Some sources say that Kay was a Saxon, but was unlike the heathen Saxons because he was a Christian.
There are different accounts of his death: throughout Welsh literature it is claimed that he was killed by Gwyddawg who was, in turn, killed by Arthur; but he is also said to have been killed by the Romans or in the war against Mordred.