In Arthurian legend, Sir Kay // (Welsh: Cai, Kai, or Kei, or Cei; Latin: Caius; French: Keu; French Romance: Queux; Old French: Kès or Kex) is Sir Ector's son and King Arthur's foster brother and later seneschal, 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 behavior, 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.
Role in Welsh tradition 
Cai or Cei is one of the earliest characters to be associated with the Arthurian cycle, appearing in a number of early Welsh texts, including Culhwch ac Olwen, Geraint fab Erbin, Iarlles y Ffynnon, Peredur fab Efrawg, Breuddwyd Rhonabwy, Pa Gur yv y Porthur and the Welsh Triads. His father is given as Cynyr, his son as Garanwyn and his daughter as Kelemon.
Before Cai's birth, Cynyr Ceinfarfog 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 Menestyr, who is in turn killed in vengeance by Arthur.
Pa Gur yv y Porthur 
One of the earliest direct reference to Cai can be found in the tenth century poem Pa Gur, in which Arthur recounts the feats and achievements of his knights so as to gain entrance to a fortress guarded by Glewlwyd Gafaelfawr, the titular porter. The poem concerns itself largely with Cai's exploits:
- Prince of the plunder,
- The unrelenting warrior to his enemy;
- Heavy was he in his vengeance;
- Terrible was his fighting.
- When he would drink from a horn,
- He would drink as much as four;
- When into battle he came
- He slew as would a hundred.
- Unless God should accomplish it,
- Cei's death would be unattainable.
- Worthy Cei and Llachau
- Used to fight battles,
- Before the pain of livid spears [ended the conflict].
- On the top of Ystarfingun
- Cei slew nine witches.
- Worthy Cei went to Ynys Mon
- To destroy lions.
- Little protection did his shield offer
- Against Palug's Cat".
Culhwch ac Olwen 
Culhwch's father, King Cilydd son of Celyddon, loses his wife Goleuddydd after a difficult childbirth. When he remarries, the young Culhwch rejects his stepmother's attempt to pair him with his new stepsister. Offended, the new queen puts a curse on him so that he can marry no one besides the beautiful Olwen, daughter of the giant Ysbaddaden. Though he has never seen her, Culhwch becomes infatuated with her, but his father warns him that he will never find her without the aid of his famous cousin Arthur. The young man immediately sets off to seek his kinsman. He finds him at his court in Celliwig in Cornwall and asks for support and assistance. Cai is the first knight to volunteer to assist Culhwch in his quest, promising to stand by his side until Olwen is found. A further five knights join them in their mission.
They travel onwards until they come across the "fairest of the castles of the world", and meet Ysbaddaden's shepherd brother, Custennin. They learn that the castle belongs to Ysbaddaden, that he stripped Custennin of his lands and murdered the shepherd's twenty-three children out of cruelty. Custennin set up a meeting between Culhwch and Olwen, and the maiden agrees to lead Culhwch and his companions to Ysbadadden's castle. Cai pledges to protect the twenty-fourth son, Goreu with his life.
The knights attack the castle by stealth, killing the nine porters and the nine watchdogs, and enter the giant's hall. Upon their arrival, Ysbaddaden attempts to kill Culhwch with a poison dart, but is outwitted and wounded, first by Bedwyr, then by the enchanter Menw, and finally by Culhwch himself. Eventually, Ysbaddaden relents, and agrees to give Culhwch his daughter on the condition that he completes a number of impossible tasks (anoethau), including hunting the Twrch Trwyth and recovering the exalted prisoner Mabon ap Modron.
Cai is a prominent character throughout the tale and is responsible for completing a number of the tasks; he kills Wrnach the Giant, rescues Mabon ap Modron from his watery prison and retrieving the hairs of Dillus the Bearded. However, when Arthur makes a satirical englyn about Cai, he grows angry and hostile towards the king, ultimately abandoning the quest and his companions. The narrative tells us that Cai would "have nothing to do with Arthur from then on, not when the latter was waning in strength or when his men were being killed." As a result, he did not take part in the hunt for Twrch Trwyth.
Other Appearances 
In the Life of St. Cadoc (c.1100) Bedwyr is alongside Arthur and Cai in dealing with King Gwynllyw of Gwynllwg's abduction of St. Gwladys from her father's court in Brycheiniog. Cai appears prominently in the early Welsh version of Tristan and Isolde, in which he assists the two lovers and is himsef infatuated with a maiden named Golwg Hafddydd, and in the early dialogue poems relating to Melwas' abduction of Gwenhwyfar. The context suggests that Cai is rescuing the queen from the otherwordly suitor, and may imply a romantic relationship between Cai and Gwenhwyfar.
The Welsh Triads name Cai as one of the "Three Battle-Diademed Men of the Island of Britain" alongside Drystan mab Tallwch and Hueil mab Caw. In the Triads of the Horses, his horse is named as Gwyneu gwddf hir (Gwyneu of the Long Neck) According to tradition, Cai is intimately associated with the old Roman fort of Caer Gai.
In the Welsh Romances (specifically Owain, or the Lady of the Fountain and Peredur son of Efrawg), Cai assumes the same boorish role he takes in the continental romances. However, manuscripts for these romances date to well after Chrétien de Troyes, meaning that Cai as he appears there may owe more to Chrétien's version of the character than to the indigenous Welsh representation.
Sir Kay 
Kay is ubiquitous in Arthurian literature but he rarely serves as anything but a foil for other characters. Though he manipulates the king to get his way, his loyalty to Arthur is usually unquestioned. In the Vulgate Cycle, the Post-Vulgate and Thomas Malory's Le Morte d'Arthur, Kay's father Ector adopts the infant Arthur after Merlin takes him away from his birth parents, Uther and Igraine. Ector raises him and Kay as brothers, but Arthur's parentage is revealed when he draws the Sword in the Stone at a tournament in London. Arthur, serving as squire to the newly-knighted Kay, is locked out of the house and can't get to his brother's sword and uses the Sword in the Stone to replace it. Kay shows his characteristic opportunism when he tries to claim it was he that pulled the sword from the stone, making him the true King of the Britons, but he relents and admits it was Arthur. He becomes one of the first Knights of the Round Table and serves his foster-brother throughout his life.
Kay's father is called Ector in later literature, but the Welsh accounts name him as Cynyr Fork-Beard. In Erec and Enide, Chrétien de Troyes mentions he had a son called Gronosis, who was versed in evil, while the Welsh give him a son and daughter named Garanwyn and Celemon. Romance rarely deals with Kay's love life, an exception being Girart d'Amiens' Escanor, which details his love for Andrivete of Northumbria, whom he must defend from her uncle's political machinations before they can marry.
Kay in later legend 
Kay and Bedivere appear in Geoffrey of Monmouth's Historia Regum Britanniae, and aid Arthur in defeating the Giant of Mont Saint-Michel. Geoffrey makes Kay the count of Anjou and Arthur's steward, an office he holds in most later literature.
In the works of Chrétien de Troyes, Kay assumes the characteristics he is most associated with today. He retains his hot-headedness and fiery temper from Welsh literature, but he is more or less an incompetent braggart. Chrétien uses him as a scoffer and a troublemaker; a foil for heroic knights like Lancelot, Ywain, or Gawain. He mocks the chivalric courtesy of Sir Calogrenant in Yvain, the Knight of the Lion, and he tricks Arthur into allowing him to try to save Guinevere from Maleagant in Lancelot, the Knight of the Cart, which ends in his humiliating defeat. In Perceval, the Story of the Grail, Sir Kay grows angry with Perceval's naïveté and slaps a maiden who says he will become a great knight; Perceval avenges her later when he breaks Kay's shoulder. Wolfram von Eschenbach, who tells the same story in his Parzival, asks his audience not to judge Kay too harshly, as his sharp words actually serve to maintain courtly order.
Scholars have pointed out that Kay's scornful, overly boastful character never makes him a clown, a coward or a traitor, except in the Grail romance Perlesvaus, where he murders Arthur's son Loholt and joins up with the king's enemies. This strange work is an anomaly, however, and Kay's portrayal tends to range from merely cruel and malicious, as in the Roman de Yder or Hartmann von Aue's Iwein to humorously derisive and even endearing, as in Durmart le Gallois and Girart d'Amiens's Escanor.
Oddly, given his ubiquity, Kay's death is not frequently dealt with. In Welsh literature, it is mentioned he was killed by Gwyddawg and avenged by Arthur. In Geoffrey of Monmouth and the Alliterative Morte Arthure, he is killed in the war against the Roman emperor Lucius, while the Vulgate Cycle has him die in France, also in battle against the Romans.
Modern adaptations 
Kay is a main character in the first three books of T. H. White's The Once and Future King, The Sword in the Stone and The Queen of Air and Darkness. His portrayal is based on Malory's account of Arthur's upbringing, but White adds a number of new elements to the story, including one in which the young Kay kills a dangerous griffin with the aid of Robin Hood and Maid Marian. White's Kay is quick-witted and often mean, but always a loving foster brother to Arthur, whom he calls "the Wart". Kay appears in the 1963 Walt Disney Studios film adaptation of The Sword in the Stone, in which he is outwardly boorish and bitter but still loves Arthur all the same.
Kay is the main character of Phyllis Ann Karr's 1982 novel The Idylls of the Queen. Expanding on a scene from the classic tales in which a knight is poisoned at Guinevere's feast and the queen is accused of the crime, Karr turns her story into a murder mystery with Kay as the detective attempting to discover the truth.
In Thomas Berger's 1978 Arthur Rex: A Legendary Novel, Kay is a somewhat foppish, sharp-tongued gourmand. Relieved to be freed from his bucolic upbringing in Wales, he takes charge of the kitchens at Camelot and yearns to make it a more sophisticated court. Arthur good-naturedly complains that Sir Kay is always serving him rich foods, when the king would rather just have simple meals. Kay supplies occasional comic relief in the book, but ultimately fights and dies with honor in the last battle against Mordred's host.
In the 1970s HTV-series Arthur of the Britons, Kay (spelled Kai) was played by Michael Gothard. In this version of the legend, Arthur is a Celtic chieftain and Kai is a Saxon orphan, raised together as brothers by an adoptive father, Llud, amongst the Celts. He is portrayed as somewhat hot-headed and sometimes distracted by female company, but a fiercely capable warrior (sometimes favouring an axe-weapon) and Arthur's most trusted and loyal friend.
In Stephen Lawhead's Pendragon cycle Kay, spelled Cai, is Arthur's most loyal companion. As a child he had a crippled leg and Arthur was one of the few who defends him. This earns Arthur his complete and unquestioning loyalty. He dies in the battle against Mordred.
In the French comedy series Kaamelott, Kay is portrayed as a rogue centurion and rival of Arthur named Caius who refused to follow his troops back to Rome and therefore celticized his name and was given knighthood by Arthur to end their rivalry.
In the 2011 Starz TV series titled Camelot (after the highly successful British series Merlin aired on BBC One), Kay is played by Canadian actor Peter Mooney. In this is an American-Irish-Canadian adaption of Arthurian Legend, Kay is portrayed as a loyal and protective older brother to Arthur. Although raised in a rural setting, he appears educated and somewhat idealistic, described by the actor who plays him as having "a world of book-smarts, but no practical experience [of how be a warrior]".
- See Rachel Bromwich's discussion in the "Notes on Personal Names", part of her edition of Trioedd Ynys Prydein: The Welsh Triads, second edition (Cardiff: University of Wales, 1978), pp. 303-307.
- Gantz, Jeffrey. The Mabinogion.
- Pa Gur
-  Trystan and Esyllt.
-  The Dialogue of Melwas and Gwenhwyfar
- Bromwich, Rachel. Trioedd Ynys Prydein.
- Bromwich, Rachel.  Celtnet.
- Starz channel promotion "Camelot: Focus on Kay"