Kunoichi were maids or servants paid by a ninja to help them infiltrate a castle and to collect information. Their main job was finding legitimate service positions in the households of enemies, to accumulate knowledge by gaining trust or eavesdropping on conversations. Their appearance can be found in the ninja scroll, "Bansen-shukai (萬川集海) ", written in Edo-period. Kunoichi means "nine plus one". The interpretation of this phrase is that, a woman biologically has "ten" = "nine plus one" holes in her body comparing with a man who has nine holes in his body like eyes, ears, mouth etc. From the casual conversations with other maids and servants, they would learn secrets.
After they were trusted by the people around them, they would ask to receive a package from their parent's home. The package would be in a large case. In the case, a Ninja would be hiding. This allowed a Ninja to sneak into the hostile castle.
Maids are given vacations to go back to their hometown a few times a year. This would be the perfect opportunity for the Kunoichi to tell their masters about all the secrets they had learned. Kunoichis would sometimes sleep with their enemies to try to learn their secrets. Kunoichis may even be able to manipulate their enemies using sex. The problem is that, Kunoichis sometimes take a liking to their enemies. It is said that for such a situation, Kunoichis needed to be watched.