Karai came from Japan to New York City to unify the warring factions of the Foot Clan that had risen following the death of the Shredder. The Shredder's Foot Elite forces had been randomly attacking anybody seemingly without reason. Karai offers the Turtles a deal; if they assist in stopping the Elite, she will give them a peace treaty with the Foot.
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History
Karai first appeared in TMNT: City at War, part 3.