Karai began her life in The Foot Clan in Japan as a typical Foot Soldier, but she quickly went through the ranks to become a premier member earning the honor of regaining control of and heading the New York City branch of The Foot.
Karai first appeared in the TMNT Universe during the City at War storyline at the end of Vol. 1.
Different fractions of the foot had been created after the Shredder's death with some still being loyal to the Foot or their former leader. Also, many gangs tried to take advantage of the tumultuous time.
Karai was ordered from the Foot Branch in Japan to lead the New York City branch and restore order. In order to do so she teamed up with the foot's arch enemies the Teenage Mutant Ninja Turtles to eliminate the Elite Guard of Foot who remained loyal to The Shredder. During this time, the loyalists targeted Karai's only daughter and killed her. In a climatic battle at April's former antique store, the Turtles and Karai eliminate all of the loyalists after tricking them into following her while she was dressed up in Shredder's armor.
The truce with the Turtle's lasted for quite awhile, but there was apparently some double-crossing and fighting that happened in the 15 year gap between TMNT Vol. 2 and Vol. 4 where Karai may have possibly gone back on her oath and attacked the turtles.
Currently, in Vol. 4, the truce has been reinstated between the turtles and Karai despite uneasy tension. The Foot were commissioned by the Utroms to be their protection when the Utroms first made public, alien contact with Earth. However, secret warriors with a special sword and dressed up in Mayan-like armor and seemingly having no genitalia, have been targeting all branches of the foot effectively eliminating all but the New York branch. Karai and Leonardo remain on tentative terms while often acting as each others sparring partner. Leonardo has doubts over Karai's motives as she apparently lies to him about the secret warrior and an occult book.
In the possible future, Karai leads Raphael on a hunt through Japan for a baby that was seemingly stolen. The baby turned out to be Karai's great-granddaughter, and all she wanted to do was to see Raphael because she misses the "old times." Karai is seemingly around 100 years old in this story.
In the IDW series, Karai (here with the full name of Oroku Karai) is the granddaughter of Oroku Saki, who has been granted immortality by the sorceress known as Kitsune. Karai is apparently one of the higher-ranking agents of the Foot Clan, although her grandfather finds her skills in need of improvement, and passes her over for the position of second-in-command. Instead, he tells Karai to bring Leonardo to him, as he is the right-hand man Saki truly wants.
Karai's history on the 2K3 Teenage Mutant Ninja Turtles television cartoon show closely resembles that of the comic history. This is due to many of the storylines from the television show drawing inspiration from the original comics.
However, there are minor tweaks to the character in the show from the comic portrayal.
In the comics, Karai never formally knew Oraku Saki - The Shredder - and was merely his replacement to get the New York Foot Branch back in order. In the television series, it is shown that Karai grew up with Oraku Saki and was considered his daughter. This relationship was shown to be adoptive rather than biological as proven later by The Shredder actually being a rogue Utrom. There is much more vehemence between Karai and the Turtles due to her animosity over their interference and banishment of Shredder from earth. In this sense, Karai takes over the Foot in the name of vengeance for her father. She is much more angry than in her other portrayals.
CGI 2007 Film
This portrayal of Karai is very true to her comics portrayal. She is brought in to fill in as the replacement to The Shredder from Japan. She is clearly Japanese, and is soft-spoken countering her immense strength and power. She acknowledges the Turtles as potential threats to the Foot and is upfront about the need for action to be taken against the Turtles for their role in the elimination of The Shredder. This mindset stems more from her orders to do so than any personal anger towards the Turtles themselves.
Initially, she works for Winters to provide him with protection as he seeks to end his immortality. She ends up creating a truce with the Turtles in order to banish the demons and the traitorous stone soldiers back to where they came from.
Karai has no superpowers or special abilities to speak of. However, she is considered one of the most deadly characters in the TMNT-universe. Her skills are often considered greater than The Shredder's, and she is one of the few people to equal the level of skill that Leonardo is considered to be at in terms of Ninjitsu and Katana weaponry.
Karai has dedicated most of her life to studying the art of Ninjitsu, and can be considered one of the modern masters of the form. She is knowledgeable in many weapon fighting styles, but is typically depicted with a single sword.
She is incredibly clever and cunning with the great ability to improvise in tight situations. She is willing to break form in order to gain the upperhand on an opponent. Overall, Karai can be either a powerful ally or a deadly foe.