Some TMNT stuff really isn't for little kids.
Chizu is the sister of Shingen, the late kashira of the Neko clan. During a previous mission commissioned by Lord Hebi and his patron, Lord Hijiki - to steal a precious sword from the collection of Lord Noriyuki - Shingen clashed with Miyamoto Usagi, who reclaimed the sword (UY # 12). Shingen later met Usagi again when Lord Takamuro made plans to usurp the position of the Shogun for himself (UY # 13-17: "The Dragon Bellow Conspiracy"). Usagi and Shingen joined forces and - along with Gennosuke and the blind swordsman Zato-Ino - stormed Tamakuro's castle, while it fell to Chizu to inform Noriyuki of the magnitude of Tamakuro's conspiracy. During the mission Shingen died, though he still managed to destroy Tamakuro and his stockpiled black powder guns (UY #17).
Some time later Chizu was raised to the status of kashira, but her position was questioned by one of her chunin (NCOs), Gunji, questioned, especially since he did not trust a woman's leadership abilities. In addition, the position of the Neko clan was weakened by the losses suffered in the attack on Tamakuro, and to strengthen their position again, Gunji hatched a plan to kidnap the mystic Kakera. But whether by coincidence or fate, Usagi and Gennosuke had just arrived in Kakera's village, and through his powers Kakera drew the Teenage Mutant Ninja Turtles into their world as additional reinforcements. Despite their efforts, Kakera was kidnapped, but Usagi was eventually able to track down Chizu, Gunji and the sage. He arrived just in time to intervene in a plot of Gunji's, who coveted the leadership over the Neko clan, against Chizu; Chizu killed Gunji in battle, left Kakera in Usagi's care and said goodbye without a fight ... but with a kiss (UY, "Shades of Green").
Since that day, Usagi and Chizu have been entertaining a close friendship (and at least a little more on Chizu's side). Their conflicting natures as samurai and ninja hinder them in establishing a closer relationship, but they trust and help each other whenever their paths cross. However, recently Chizu fell out of favor with her clan and became a fugitive from her own people. Her current status is unknown.