Kay

Sir Kay (サー・ケー) is one of the Knights of the Round Table.

Background
Kay was the son of 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. 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.

Kay was one of the most senior knights of Camelot along with Bedivere and Gawain.

Personality
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.

Fate/hollow ataraxia
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.

Abilities
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.