One of the most vicious obstacles in Beatrix Kiddo's hitlist, O-Ren was the overlord on high of a Tokyo-based triad that counted the unhinged Crazy 88 and Gogo Yubari as members. As 'Cottonmouth' in the Deadly Viper Assassination Squad, she carried out a massacre that killed nine innocents in Texas and made Kiddo comatose. 

Ishii was orphaned after Yakuzas attacked her house when she was a child. The brainchild of that attack took her under his wing shortly thereafter, in exchange for pedophile favors. Ishii later disemboweled her warden and set out to rule Japan's Yakuza.

Ishii would seem like the finest blueprint for Nikki Green's revenge fantasies when she grows up.

Ishii herself learned from the best and more. Her adroitness in samurai swordfight and low threshold for slights make her one big bad snake.

Get a replica of O-Ren Ishii's swords now:

Fully hand-forged replica of O-Ren Ishii's sword

This one's from Heavenly Swords