Kay was the son of Sir Ector and foster brother of Artoria, who later became known as King Arthur. Due to being female, Artoria could not inherit the throne by normal means, so she was given to Ector by Merlin to be raised as a simple knight apprentice at the age of five. Although Kay could tell she would grow up to be a beauty, his father told him to guide her like she was his brother. Deciding to treat her in that manner, he did not believe it would be kept secret. Artoria was initially raised under the belief that they were related by blood, but she was later told the truth while growing up. Their relationship did not diminish from the truth, and they still felt they were true siblings. Her identity as being female was known to him, but he kept the secret all his life after being sworn to secrecy by Merlin. Artoria acted as his attendant, carrying their weapons and provisions on outings, and she trained with both Kay and Ector in the ways of combat and the ways of being a knight. They often trained with wooden swords, and although Artoria was superior, she never once defeated him. Despite her always having been victorious in combat, they always argued while they fought, and due to Kay's arguments, she always felt like she had lost against him.
One notable incident in their childhood for Artoria occurred during a rainy day while Artoria was sick and Kay's mother had to leave, so only Kay was available to tend to her. Telling her "it would be troublesome for me if you were to die now", he promised to do anything she requested in exchange for getting well quickly. He was very persistent in having her request something, so she said she wanted to "dream of a lion running across plains." He then left to carve a wooden lion, hoping to make an old, dying lion unable to hurt a mouse so as to not have it harm her in her dreams. The carving was poor enough that she compared it to a cross between a dog and a cat, but she appreciated it and was able to see the dream. Kay became shocked at his lack of carving skill upon her telling him, sadly telling her his original intention for it.
He was not there to witness her pulling the sword from the stone, but felt no need to warn her off her course due to his indignation toward the situation for she who "had everything since she was born." Afterwards while mastering the sword, she, Merlin, and Kay went through various adventures while training. Eventually calling herself the rightful King of the country, they established their base of operations and eventually fought eleven battles to claim the country. Kay was one of the most senior knights of Camelot along with Bedivere and Gawain, and the most original Round Table consisted of only himself, Artoria, and Merlin. When Artoria lost Caliburn to the machinations of Morgan, he made her a wood carving of a bird catching a salmon in an attempt to cheer her up. After the final battle against Vortigern, she claimed her kingdom and they spent ten years in peace.
Before the Battle of Camlann, Kay suspected that Mordred was preparing troops for an uprising when Artoria returns from the expedition to find Lancelot in Rome. However instead of choosing to stay and join the battle, Kay thought about leaving Camelot, taking all of his fortune and setting off for another land, despite feeling conflicted. Even so, he is mentioned as the reason why Artoria was able to disembark in Britain during the rebellion, so it's not clear if he actually abandoned the battle.
Some time after Artoria's death, Morgan le Fay created the Mystic Code Add to seal Rhongomyniad and keep it from losing its Mystery. Morgan chose Kay as the base for Add's pseudo-personality because of all the Knights of the Round Table he was the one who had the least admiration for King Arthur.
Kay is very argumentative, described as "possessing an elegant way of speaking" by Artoria, and he never lost to anyone in a battle of words. Artoria knew of no better knight when it came down to arguments, and his oratory skills were vaunted such that there were exaggerated rumors of Kay having shocked fire dragons in to flying back from whence they came. During his sparring with Artoria, he would use "warped reasoning just on the verge of being correct" to make her feel like she had lost despite being victorious, such as "You lose since you threw away your sheath!" or "I'm still alive so don't act like you won!" Once she became king, he often gave her strict and frank advice, and she believed such wit was required to rule a country. He is very skilled in management, so much that not a single shilling was wasted during any of their campaigns when he was in charge of the funds. He is fond of women, often flirting around while performing his duties as a knight.
Despite his love for Artoria, he, like many others, holds some resentments towards her for sacrificing herself. Unable to understand that for which she struggles, he finds her dedication to something she never once personally experienced to be sickening in a manner enough to make him shudder.
Upon getting into a competition with Illyasviel von Einzbern and Sakura Matou as to who is the best little sister, Saber recounts her time with Kay. When mentioning his argumentative side, the others compare him to Shinji Matou. Illya summarizes his attributes as "foul-mouthed, stingy, and weak", although Saber defends him. She also mentioned how Kay was always at odds with Merlin.
Garden of AvalonEdit
Kay's story is a one-sided narrative told to Merlin before the downfall of the kingdom. Kay mentions that he hates having to listen to the stories of others, but says he will give his best attempt to bear with telling his own should Merlin wish to listen to him. Recounting the circumstances of how Artoria came to be his "brother" and the circumstances behind her birth, he believes she was born for the purpose of making the next king something that was not "merely above other men, but something that was no man." He describes her bearing as different compared to others, that none could have grown into a normal human under the conditions of her birth. He describes her battles and finally coming to become king.
Concerned with the state of things, that none have truly accepted Artoria due to her inhuman qualities, he believes it is likely to fall apart at any time. He believes that Mordred is preparing troops for when Artoria returns from the expedition to find Lancelot in Rome, and Kay says he is tired of it all. Deciding to leave from all of it, he plans on maybe taking his fortune and setting off for another land. When Merlin asks of what Kay thinks of Artoria, he tells his frustration with her and recounts a time when she had been lacking sleep and later learned that she even spent her sleeping hours under the instruction of Merlin in her dreams. Looking upon the end of the kingdom, he can only think, “Bollocks. What was it that she wanted to do so much to have to go that far?”
Fate/Grand Order Edit
During the "Camelot" Singularity, the Lion King, an alternate version of Artoria, summons Kay along with all of the other knights with the exceptions of Bedivere and Galahad. She claims to require their service in order to perform the "Holy Selection" that will help counteract Solomon's destruction of the world. She gives the knights a half-day to ponder over serving her or going against her. Sir Kay was strangely silent and vanished till sunset without saying a word.
He decides to fight against Artoria as he couldn't agree with her decision. Although he bore no resentment, he resolved to stop his sister and was slain by the Lion King.
Curtain Call ～LET US DRIVE TOGETHER～ Edit
While in a cemetery, Kay appears before Shirou in spirit form.
Lord El-Melloi II Case Files Edit
Kay is inferior to Artoria in terms of swordsmanship, but she praises him by describing him as possessing "an understanding of his own level of skill." He was the only one of the knights Artoria had complete faith in to be entrusted with the rear guard in their campaigns.
In Fate/Grand Order's "Knights of Marines" craft essence, Kay is remarked to be the best swimmer in the Round Table. His abilities were described as "freakish" by the other knights.The Kay of Case Files is not a Servant, and he is not from the Throne of Heroes. He is the basis of the personality of Add, and he can be manifested similar to a Servant. He uses the Noble Phantasm Camelot Image: Fleeting Forgotten Castle (儚くも忘れじの城, Hakanaku mo Wasureji no Shiro?), but it is a limited "Virtual Noble Phantasm" (仮想宝具?) similar to Lord Chaldeas.
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Garden of Avalon
- ↑ 2.0 2.1 2.2 2.3 Fate/hollow ataraxia - Sisters' crisis
 Fate/side material - Encyclopedia: Sir Bedivere [Person's name], p.061 [T]
Sir Bedivere [Person's name].
The knight who tended the final moments of King Arthur.
He was the last remaining knight who returned the holy sword back to the lake according to the King's orders.
Lancelot, Percival, Galahad were some of the popular knights of King Arthur's who embarked on the quest for the Holy Grail, but they were actually junior knights who joined later for the quest.
The earliest and most famous knights who swore allegiance to King Arthur included Ser Kay, Ser Gawain, and Ser Bedivere. They had histories even older than King Arthur's.
Ser Bedivere's name was thought to be derived from the ancient War God of the Celts, Bedwyr.
By the way, Ser Bedivere was not aware that Artoria was a girl.
 Lord El-Melloi II Case Files material - Encyclopedia: Add [Mystic Code/Other], p.097
Add [Mystic Code/Other]
The Mystic Code sealing the Noble Phantasm Rhongomyniad, possessed by Gray.
Created by the Witch Morgan, she based the artificial personality within it on the knight with the least adoration for King Arthur from the Round Table. Thinking about the Atlas Contract Incident, that trick seemed more like a plot to “prevent the return of King Arthur” at that time. It was like a strange trick to say “I’ll help with your revival, but I don’t want it to succeed.”
Before being created, its true form was that of the Logos React Replica. Using the extreme calculative ability inherited from the Seven Superweapons of the Atlas Institute, it continues to seal Rhongomyniad within it. In its released form, it can of course take the shape of a scythe, but also that of a hammer, shield, hook spear, or even a boomerang. It is also able to use its immense calculative ability to display a portion of the power of Rhongomyniad itself. The appearance of the knight in “The Atlas Contract” occurred due to the same theoretical principle. Their personality was a result of a complex merger of that original knight’s and the present day Add’s, so strictly speaking it belonged to neither of them. That was likely why it was able to finally deploy the Pseudo Phantasm.
For Gray, Add was her only friend for a long time.
The name Add comes both from the word “addition” and from the tail end of the word “Rhongomyniad.”