Hamato Miwa, aka Oroku Karai, or Serpent Karai, is an ally of the Ninja Turtles. As the daughter of Hamato Yoshi and Tang Shen who was born and raised in Tokyo, Japan, until Oroku Saki assassinated her mother and took her as his own daughter to later on be renamed, become the second-in-command of The Foot Clan, become a Kunoichi, and was lied by The Shredder about her background. Years later when she accompanied The Foot to New York City for the hunt of Splinter, she later on turned her true background and started to rebel against The Foot, until she was mutated into a mutant serpent (after a failed attempt of mutating the Turtles) and was loose in the city. But once she was caught by The Foot Clan, she was brainwashed with Parasites and became forcefully loyal to The Shredder, until she was free from his control. Afterwards he recruited her old friend, Shinigami, and started a war against The Foot, which lead to her becoming the new leader after The Foot was defeated. She uses the Shikomizue as her signature weapon in combat. She is also the love interest of Leonardo. Karai debuts New Girl in Town and Serpent Karai debuts in Vengeance is Mine.
Years ago in Tokyo, Japan, a baby Japanese girl was born and raised, being given the name Hamato Miwa by her parents, Hamato Yoshi and Tang Shen. The Hamato family live in harmony with their daughter and while Yoshi was distracted with her, Shen, and The Hamato Clan (due to his father slowly dying and him being the second-in-command of his family clan), the harmony was then destroyed by Oroku Saki, the stepbrother of Yoshi. Saki who had found out his true background and claimed the throne of The Foot Clan, decided to attack Yoshi in the Hamato Dojo in the middle of the night, as he was seeking vengeance on him for supposedly taking away Tang Shen from him, the girl that he loved as well. During the fight between the two rivals, Oroku Saki was willing to assassinate Yoshi with his Tekkō-kagi in the fire that was consuming the Dojo, which was going to occur when the climax of the fight came, until Tang Shen interfered in the fight and took the final blow of the Tekkō-kagi. After she was killed and the men were devastated, Saki got burnt by a piece of burning ceiling and was permanently scarred by these burns. When he exited the Hamato Dojo that was burning down in the fire, he found baby Miwa, who was abandoned and took her as his own daughter.
Ever since that tragic night, Oroku Saki, now renamed in his birth clan as The Shredder, had renamed the daughter of Hamato Yoshi, Oroku Karai. Oroku Karai was raised by The Shredder as his own daughter and was trained by both him and Hattori Tatsu into the ways of Ninjutsu, becoming a Kunoichi later on. At some point she was given by Oroku Saki her Shikomizue and became the second-in-command of The Foot Clan, after Tatsu fled from the clan. 15 years later when Shredder and The Foot found out that Hamato Yoshi was living in New York City, she went with them to help out in the hunt for Shredder's old enemy.
After she was taken to Baxter Stockman's Laboratory, she was locked up in a cage that was on top of the Mutagen Tank of Baxter Fly, with his ultimate plan being revealed to Tiger Claw of mutating the Ninja Turtles once they got near the tank, so that they could mutate into mindless serpents with the Mindless Serpent Serum that Stockman developed. This would lead for the Turtles (now mutant serpents) to attack and kill Splinter mindlessly and from there The Shredder would finally achieve his vengeance. Once the Turtles and Splinter infiltrated the laboratory to rescue Miwa, Splinter, Raphael, Donatello, and Michelangelo were all captured, leaving Leonardo to rescue her.
Once he had reached the cage that contained Karai and was about to set her free, The Shredder out of nowhere came to attempt to push him, but his Steel Claws cuts off the chain that was holding Karai's cage. This causes the teenager to fall into the Mutagen to mutate into a mindless three-headed serpent mutant (who has shape-shifting abilities, possibly gained from the mysterious substance in the serum), which devastated the Turtles, Splinter, and Shredder.
Enhanced Strength: She is strong enough to constrict her opponent. Also she has enough strength to destroy things like Foot Bots and Kraangdroids. Another thing is that she has the strength to lift a person up like Splinter. This is even used when she is constricting someone.
Enhanced Endurance: In combat if she is attacked, she can recover fast.
Enhanced Agility: With her snake abilities, she had increased her speed. This makes her able to attack opponents in a limited time and even slither away quickly from danger.
Miwa strongly resembles Leonardo's Wife from the Mirage TMNT mini-series Tales of Leonardo: Blind Sight. In this story, Leo experiences a vision of another life where he is a turtle samurai in feudal Japan. He has an unnamed human wife, and together they have a daughter named Yumi. Leo's wife and Miwa have facial structures and hair styles that are very similar. But unlike Miwa, Leo's wife also ties her hair, wears no eyeliner or face paint, and her widow's peak is positioned higher and away from the "villainous widow's peak" style. Overall, Miwa looks like a more evil version of this character.
In one of Mutant Karai's concept arts, she is given the name of "Kiba". Even though there is no explanation as to why this name was given to her mutant form, the name hasn't been used in the series either.
The mutation that Karai undergoes has a loose resemblance to a mythical creature in Japanese folklore known as the Nure-onna (which translates to "wet woman"). This creature is a yokai that resembles a woman with a snake body that is mostly spotted near a shore.