Some TMNT stuff was always meant for grownups.
Amai is Karai's daughter, who also functioned as one of her top aides in the Foot Clan. She accompanied her mother to New York City to root out the loyalists to Oroku Saki. During the event dubbed "City at War", Amai was attacked and thought to have been killed. However, she instead suffered brain death. Karai quit the Foot Clan to care for her daughter, although she also secretly was sending out ninja to dispatch the last of the Saki loyalists.
As it turned out, the "Amai" in the bed next to Karai was actually the morphling Lurch, and the nanotech had instead taken over the braindead Amai as a host, turning her into the Lady Shredder.
After Raphael fought Pimiko, the daughter of the Shredder as she attempted to wrest control of the Foot Clan from him, a female clad in Shredder armor appeared. Raphael, thinking it was Pimiko, conceded that she had proven herself worthy of taking back the clan, but he then saw her, complaining about an injury she suffered during the fight.
Lady Shredder then chained up Raphael, forcing him to join her rule of the Foot, or die. The other Turtles arrive and free him, and the four of them along with Pimiko battle Lady Shredder and the Foot.
Later, the Lady Shredder accompanies Go-Komodo to "Amai's" (Lurch's) hospice room, to act as his backup as he tries to force his niece Pimiko to rejoin him. Lady Shredder states that the armor had developed its own AI, leading Leonardo to believe that the armor is empty. However, when Karai kicks off her helmet in order to protect her "daughter", she realizes that the armor has taken over her actual vegetative daughter.
All events prior to her debut in Urban Legends #24 took place during the Mirage continuity, which Image continuity was originally meant to be canon with and is an offshoot continuity. In the original Mirage continuity, Karai's daughter remains unnamed and is never mentioned after her death. Since Karai remained with the Foot in this incarnation, this gives the idea that she did not survive, unlike Amai here.
- Amai (甘い) is Japanese for sweet, which contrasts her mother's name which can mean salty, spicy, or bitter.
- Image's series had ended before Lady Shredder's identity could be revealed. A note in the comic's letters pages indicated that it was intended to be Karai. In Andrew Modeen's quasi-canon Teenage Mutant Ninja Turtles issue 24 (Image) and issue 25, she is revealed to be Headhunter/The Mistress, the sister of the late Tang Shen and the Shredder's consort.