Karai is the Japanese Foot Clan leader, and has came to take over the New York Foot after the Shredder's death, acting as mercenaries. She was hired by the mysterious billionaire Max Winters to be "the eyes and ears" of the city, though she showed anger later when she found the truth of his unexpected order to hunt down Monsters.
During the tide of the battle, both the Foot Clan and the Turtles realized that Winters, revealing himself to be an immortal warlord named Yaotl, only wanted the monsters to be returned to their world to undo the immortality curse he and his Stone Generals had endured. Unfortunately, the Stone Generals betrayed Winters by deliberately missing the final monster and intended to use the portal to finalize their conquest of Earth. When the Stone Generals offer the Foot a chance to join them in world conquest, Karai refuses, saying that they would honor their contract to Winters. Aided by April and Casey, she then leads her ninja in retrieving the last of the monsters. In the end, Karai parts ways with the Turtles peacefully, but gives a cryptic last word that Turtles "have passed" something (she refuses to elaborate) and "soon we will have further business together; the kind that involves familiar faces from your past", hinting on the Shredder's upcoming return, which was indeed planned for the film's sequel, but was canceled because Peter Laird sold the franchise to Viacom.
She briefly fought with Leonardo, rejecting his offer to help them, implying that she is aware of his role in the past events involving the Foot. This is supported by her line to the Turtles, "you are every bit as warriors I was informed you were" after the defeat of the Stone Generals. Karai also seemed to have had prior knowledge about Leonardo; before their brief confrontation Karai proclaimed 'You!' as she faced him. Karai also engages in battle with April O'Neil while the Foot battles the Turtles and their allies.
She is shown wearing a full-body black armor with a hood on her head and a short cape (if noticed carefully, there's a hexagonal ruby in the center of her chest).
This version of Karai may or may be not related to the Shredder; in the bio text of her action figure in the film's merchandise she is described as being 'the Shredder's daughter', recalling her character of the 2003 series, but no clear indication of this was given in the actual film.