"Yes Father, I used the matter transporter to rescue the Turtles from your scanner!" - Karai finally betrays the Shredder
Karai was the adopted daughter of Ch'rell also known as Oroku Saki, and the Shredder.
Although raised by Ch'rell she wasn't evil like him, she was just loyal to Ch'rell and would follow him no matter what he did, she torn between honor and family.
She appears throughout the TMNT series both as an ally and enemy. After Ch'rell's exilie for a while she became the Shredder herself. In the end she finally betrays her evil Father and joins the Turtles.
As a child, Karai was found by Oroku Saki after her parents abandoned her. He took her in and raised her as his own, teaching her Bushido. She became his most trusted follower, even trusted to his true identity as Ch'rell and told her that the Utroms were savagely hunting him. When the Utroms fled for New York, Karai remained in charge of the Japanese Foot Clan.
When the Purple Dragons, the Mafia, and the Foot were fighting for control of New York after the Shredder's demise, Karai arrived in the city and quickly enlisted the aid of the Turtles to stop it, promising an end to the Foot’s vendetta against them in exchange for their assistance. Leonardo, believing that Karai was honorable despite her legacy, convinced Donatello and Michelangelo to aid Karai. Raphael refused, although he later recanted. Together, Karai and the Turtles eventually retook control of the New York Foot and consequently its underworld. However, the end of the story revealed that Karai had been manipulating the Turtles, and had been working with the not-dead Shredder all along. However, she showed sadness when the Shredder (Chrell) vowed to get revenge on the Turtles after she had made a truce with them.
When a fully-healed Ch'rell retook control of the Foot. As his first order of business, he deployed a killer robot (disguised as Splinter) to eliminate the turtles. Realizing that their agreement with the Foot had been recanted, the turtles decided to face the Foot again in their temporary HQ—a ship stationed at the docks. Leonardo, Raphael, and Splinter were taken hostage during their encounter, and Karai was forced to fight Leonardo. Despite Leonardo’s lack of resistance, she proved unwilling to kill him. Karai’s internal conflict—her loyalty to the Turtles as fellow Bushido-practitioners vs. her loyalty to her father—was the main thrust of her Season 3 storyline, as she was pulled from both directions to take a side. She managed to stay in the graces of both sides until the Turtles faced the Shredder again in the season’s finale. As the Turtles were chasing Ch'rell Karai tried to reason with Leonardo, pleading him to let her father leave. When Leonardo refused to let Shredder go, Karai fought alongise with his father and (unintentionally) stabbed Leonardo in the shoulder with her sword, damaging his carapace, although she later prevented the Utrom Shredder from killing the Turtles while they were unconscious. Despite her efforts, however, the Shredder was taken prisoner and exiled by the Utroms, and Karai and Foot Tech Division Head Dr. Chaplin were sent back to Earth with Karai crying out to Leonardo: "This is not what I wanted!"
As the ShredderAfter her Father Ch'rell was exiled by the Utroms she vowed to take revenge. Karai took up the mantle of the Shredder and wore her own Shredder like armour.
Furious at what she saw as a betrayal by the Turtles, Karai plotted her revenge against them, first attacking them on the psychic plane via Foot Mystics, and later coordinating and leading an assault against them at their lair, which she believed successful. She later on faced Leonardo (who was not at the lair at the time of the assault), who defeated her but then departed with the warning to leave his family alone and giving her one last chance for redemption.
Karai next faced the Turtles during their attempt to retrieve a Foot artifact, an amulet called the Heart of Tengu. Despite her efforts, she was unsuccessful in stopping them. Unknown to the Turtles or Agent Bishop leader of the Earth Protection Force who initially charged them with retrieving the artifact, the Heart of Tengu was a mystical artifact that allowed the Foot to control its Elemental Mystics, who were in reality the heralds for a man-demon that had centuries ago terrorized Japan as The Tengu Shredder
Return of the Original Shredder
When Karai realized that the Original Tengu Shredder was about to return, she had Dr. Chaplin produce some new mystic weaponary that could be used to fight the Tengu Shredder. When the Mystics ressurected their old Master, the Tengu Shredder then launched a full force attack on Karai's HQ. Easily defeating the Foot ninja, Foot robotic soldiers and weaponary and even Ch'rell's elite guard.
When the Tengu Shredder and his Mystics reached the top floor of the building where Karai was, Karai wearing her Shredder armour and armed with soom mystic weapons Chaplin gave her she trys to fight the Tengu Shredder, but not even her skill and the training Ch'rell gave her helped her. She manages to slow the Tengu Shredder down by throwing a case of a Mystictical substance on him and even stabs him with her katana. However the Shredder is not dead, he changes his face back into his mortal form which is identical to Ch'rell human exo suit face. Karai believes it is her father, but he quickly transformes his sword into a snake which attacks Karai, with Karai out of action the Shredder thinks about making her his slave and strips her of her armour. Luckily the Turtles arrive who managed to defeat the Foot Mystics.
The Shredder then turns the top of the tower into a graveyard. His minions had Karai still, and he proceeded to strike her when Chaplin tried to defend her. The Shredder could instantly see that he was in love and decided to cut it short, wounding Karai. The Turtles summoned their dragon avatars to battle the Shredder, sending him and the Mystics plunging down to the ground. As they retreated, the Shredder looked at the modern world and announced his plans to modify it for his own.
t was soon learned that because she shared The Shredder's mantle, Karai had the latent psychic ability to lower Shredder's defenses through a shared link. This was accomplished by absorbing the majority of the demon shredder's power into herself, a power that she presumably keeps. This gave the Turtles an advantage in their final conflict with the demon. Karai helped pave the way for her namesake's destruction, and Dr. Chaplin helped the Turtles co-ordinate their battles against the demon. After the battle, Karai, touched by Chaplin's devotion to her, promised him that she was going to worry more about the immediate future, and they departed the battlefield hand in hand. It is possible that after it she forgot her grudge against turtles and resolved their difficulties off-camera.
She and Dr. Chaplin later attend the wedding of Casey and April the friends of the Turtles.
In Turtles Forever, Karai freed Ch'rell from the clutches of the 1987 Shredder, Krang, Bebop, and Rocksteady, and aided the Utrom Shredder in modifying the Technodrome with Utrom technology. The Utrom 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 Utrom Shredder was prepared to destroy both the 1987 and 2003 Turtles.
Ch'rell trys to fight the Mirage Turtles of Turtle Prime, but then the 1987 and 2003 Turtles appear and Ch'rell realizes that Karai saved them. The twelve Turtles, with the aid of Splinter, Karai, and even Saki and Krang, engage Ch'rell in battle, but he grows to massive height using molecular amplification technology from Dimension X. When Ch'rell is accidentally clipped by the energy beam from the Technodrome, however, his armor is damaged, and the Turtles all try to force him into the beam ... before it is abruptly cut off when Rocksteady trips over the power cable and unplug it. Ch'rell takes this opportunity to grab the four Mirage Turtles and begins crushing them. At the last minute, the 1987 Turtles throw explosive throwing stars at him, causing him to trip and drop the Mirage Turtles. Ch'rell then resumes his attack - until Bebop plugs the beam power cable back in and thus inadvertently obliterates Ch'rell.
Karai then leaves Turtle Prime with the Turtles and Master Splinter while the 1987 Turtles, Shredder and Krang leave on the Technodrome. Karai has now once again become an ally of the Turtles.