Kai is the seneschal of King Arthur, or Messire Queux as he's known in the French romances. He is also sometimes called Maitre Queux or Kuex. He featured heavily in the 'Morte d'Arthur'. The tale of Kilhwuh and Olwen in the Mabinogion he is described as a person whose "breath lasted nine nights and days under water" and who "could exist nine nights and days under water" and who "could exist nine nights and nine days without sleep".
There was no way to cure a wound caused by his sword and he had the ability to make himself as tall as the tallest tree. His nature was so hot that anything he carried in rain stayed dry. Once a deity he degenerated through several mythological episodes into a hero.