The origin word kunoichi is worthy of mention. It is japanese for female ninja and, in a roundabout kind of way, woman. It's written in all three Japanese alphabets; hiragana, katakana and kanji. When you combine the different characters, it forms the kanji for woman.
Some people claim that ninja clans would adopt and young orphaned girls and raise them to become kunoichi. Apparently, they would be so grateful for being adopted (otherwise they'd probably have died) that they would be willing to do anything for their clan. Even though there is some logic to this theory, it wouldn't seem entirely out of place in a cliche martial arts movie. As of yet I have found nothing to prove or disprove it.
Although there are those who say that the average kunoichi could hold her own in a physical battle, most ninja enthusiastes agree that they probably stuck more subtle methods of espionage. One such method was to infaltrate the court of a powerful lord, often by marriage. Being privy to the gossip and politics of an enemy's home and having an influential husband could prove to be useful, as could direct access to his food and drink and an extensive knowledge of poisons.
Kunoichi never captivated western audiances the way their male counterparts did, but they were a vital part of a ninja's information network. While they weren't directly involved in the action male ninja took part in or in charge of some of the more romanticized aspects of a ninja's job, but their work needed to be done. It's unlikely that the men they worked with would have been nearly as successful in some of their missions without the women backing them. Kunoichi were undeniably indispensable.