Karai is a member and leader of the Foot Clan Japanese branch, and, following the events of City at War, became the leader of the Foot Clan New York branch.
Karai (辛い) is a Japanese word commonly used to describe the flavor "spicy/hot". However, karai can also mean "extremely severe", "harsh", "strict" or "storm". (The word also shares the same kanji (辛) as the word tsurai which means "difficult", "painful", or "cruel".)
She was hired by 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. She briefly fought with Leonardo, rejecting his offer to help them, implying that she is aware of his role in the past. 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.
When the Stone Generals offered the Foot a chance to join them in world conquest, Karai refused, saying that they will honor their contract to Winters. She led her ninja in retrieving the last of the monsters (aided by April and Casey). Karai parted ways with the Turtles peacefully, after giving a cryptic last word that "savor your victory tonight, for soon we will have further business together; the kind that involves familiar faces from your past", perhaps hinting the Shredder's return.
She was described in her toy bio as being 'the Shredder's daughter', recalling her character of the 2003 series. However, no indication of this was given in the actual film.
Karai freed Ch'rell from the clutches of the 1988 Shredder, Krang, Bebop, and Rocksteady, and aided the Shredder in modifying the Technodrome with Utrom technology. The Shredder, consumed with rage for revenge on the Turtles, wanted to destroy every dimension where Ninja Turtles exists, even though such an action would most likely destroy himself. Karai began to have doubts about the whole affair and secretly intervened as the Shredder was prepared to destroy both the 1988 and 2003 Turtles. In the end, at the Turtles Prime dimension, (Mirage Turtles) Karai fought alongside the Mirage, 1988, and 2003 Turtles as well as the 1988 Shredder and Krang and 2003 Splinter, to stop the Shredder from destroying Turtles Prime. In the end, after Ch'rell was defeated, she commented that the Shredder would always return.
A major plot arc that started with her character during Season 2 of the show was that she was loyal to her father, despite his dishonorable ways. After Ch'rell's exile at the end of Season 3, Karai spent all of Season 4 and Season 5 hating the Turtles and possibly after season 5 making truce with Turtles. When she appeared in 'Wedding Bells and Bytes' as a guest of April & Casey's wedding, it was assumed that her vendetta with the Turtles was over, but Turtles Forever (if it is meant to fully correspond with the 2003 series' plotline) indicates that with Ch'rell's return, her loyalty to him does not exceed her truce with the Turtles. But duringTurtles Forever she finally betrayed The Shredder and became friend of Turtles once again.