Sir Kay

Sir 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. 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.