Mochizuki Chiyome and the Kunoichi
Ever since the West has been in contact with the Japanese, the ever powerful, mysterious, and seemingly magical ninjas have captured Western imagination. Ninjas were a secret group of special forces, high trained, spies in Medieval Japan. Although popularized today, traditionally ninjas commonly did not wear the all black costume that we imagine them in, instead they were usually found to be dressed like everyone else or as religious lay folk.
In the 16th century, there was a Japanese noblewoman by the name of Mochizuki Chiyome who was rumored to be from the Koga clan (a Japanese clan that focused on the art of ninjustsu) and was married to a samurai warlord, Mochizuki Nobumsasa. While he was off in battle, Chiyome was left in the care of a daimyo (a warlord) Takeda Shingen, who was her husband's uncle.
(Edit: There is little known about Chiyome, only that she appears briefly in records, and then is never heard about again after Shingen's death. If you've heard about her before, its either because you really like history, or you might have played or seen her in a video game.)
After her husband died in 1575 at the battle of Nagashino, Shingen tasked Chiyome with creating an network of female ninjas who could gather information and deliver coded messages without being suspected. Chiyome began her base of operations in the village Nazu, in the Shinshu region, and began recruiting. She focused on young women who were recently orphaned, lost, abandoned, prostitutes, or victims of the civil wars.
It was believed that she was helping these girls, giving them a new chance at life, but really, she was training them to become highly efficient information gatherers and verifies, seductresses, messengers, and assassins. They were all taught the skills of female shamans or shrine maidens (called a miko) so that they could travel basically anywhere without suspicion. They also learned how to disguise themselves as actresses, prostitutes, or geisha. But mostly they went to work as servants or ladies-in-waiting where they could gather gossip and build connections.
Overtime, Chiyome developed a network of over 200-300 female agents who worked for the Takeda Clan. Shingen became very well informed but eventually died mysteriously in 1573.
Mochizuki Chiyome - Badass of the Week (This was my favorite and most entertaining resource)