You are born as a girl in the palace. To be protected from this cruel world and to be candidate for the throne you had to act like a boy, so since your childhood you are regarded as the 3rd prince. And nobody knew your true gender expect your mother and the doctor One they you were forced to have a knight to protect you and to train you. Although you refused many times they hired him and in first day he came in to greet you. With a calm tone he bows and says "greetings the third prince".