Kay is an Unknown-class Servant summoned by the Goddess Rhongomyniad in Fate/Grand Order's Grand Orders.
He's best known as the foster brother of Artoria, who would eventually become King Arthur. Since Artoria couldn't inherit the throne as a woman, Merlin arranged for her to be raised by Sir Ector—Kay's father—as a simple knight apprentice starting at age five. Even though Kay could see she'd grow into a beauty, his father told him to treat her like a brother, and he took that instruction seriously.
Artoria grew up believing they were actually blood-related, though she learned the truth later on. The revelation didn't change anything between them—they still felt like true siblings. Kay kept her secret gender under wraps his whole life after Merlin swore him to secrecy.
As they trained together, Artoria served as his attendant, hauling their weapons and gear while they practiced combat with wooden swords. She was the better fighter by far, but she never actually beat him—at least not in a way that stuck. The thing is, they'd constantly bicker and argue while sparring, and Kay had a way of getting under her skin that made her feel like she'd lost every time, even when she'd clearly won. That dynamic pretty much defined their entire relationship.
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