Which of the Knights of the Round Table was King Arthur's foster brother?

In Arthurian legend, Sir Kay (Welsh: Cai) is King Arthur's foster brother as well as one of the first Knights of the Round Table. In later literature he is known for his acid tongue and bullying, boorish behaviour, but in earlier accounts he was one of Arthur's premier warriors. Along with Bedivere, with whom he is frequently associated, Kay is one of the earliest characters associated with Arthur. Kay's father is called Ector in later literature, but the Welsh accounts name him as Cynyr Ceinfardqfog.

Before Cai's birth, Cynyr prophesied that his son's heart would be eternally cold, that he would be exceptionally stubborn, and that no one would be able to brave fire or water like him. Cai is attributed with a number of further superhuman abilities, including the ability to go nine days and nine nights without the need to breathe or to sleep, the ability to grow as "tall as the tallest tree in the forest if he pleased" and the ability to radiate supernatural heat from his hands. Furthermore, it is impossible to cure a wound from Cai's sword. Cai is killed by Gwyddawg fab (son of) Menestyr, who is in turn killed in vengeance by Arthur.