“Julie D’Aubigny was a 17th-century bisexual French opera singer and fencing master who killed or wounded at least ten men in life-or-death duels, performed nightly shows on the biggest and most highly-respected opera stage in the world, and once took the Holy Orders just so that she could sneak into a convent and bang a nun. If nothing in that sentence at least marginally interests you, I have no idea why you’re visiting this website.”
There’s also the bit where she and her nun girlfriend escaped by burning down the convent to fake their own deaths. And her habit of flashing anyone who didn’t believe such a good swordsman could actually be a woman. And the boyfriend she picked up by driving a foot of sharpened steel through his shoulder.
Basically, what I’m saying is that if she weren’t real, I’d accuse Steven Moffat of making her up.