By virtue of being a woman, she was forbidden to become one of them. Her twin brother Charles had been raised knowing he would one day join their ranks. From practically the age of birth, he had been groomed to grow into the role. She was able to train with him only because their father was gone far too often to put an end to it.
Their father died in battle at virtually the same time Charles succumbed to the most recent outbreak of the plague. Only she remained, 7 years old and alone in the world. Of course she would assume Charles' identity. By the age of 19, when she had finished her years of acting the page and squire, the accolade took place as scheduled. Sir Charles, she was. Only her childhood friend, Thomas, a fellow knight, knew the true "Charles."
In the end, the inevitable happened, as it always does in tales. Months after Thomas and "Charles" attested their love for one another, the bump in the knight could not be hidden and her true identity was revealed.
[Yeah, yeah, yeah. I know. Go read about some true "bumps in the night" over at Sunday Scribblings.]