The Yuan Yuan are pirate scrap dealers without a country that offer themselves as mercenaries. They are outlaws, despised by a society that holds its nose long enough to use them as hired cannon fodder. The Yuan Yuan are good brawlers who join these undisciplined, loud units for the money.
Including a Yuan Yuan in your army is undoubtedly the way to guarantee an unpleasant surprise for your enemy. Deploy her in the midst of the enemy lines and enjoy the chaos this generates among their troops.