Some TMNT stuff really isn't for little kids.
Kitsune is a shape-shifting witch from the era of feudal Japan, who played an integral part in the Foot Clan's rise to prominence. She is also the youngest member of the Pantheon, and a such is a daughter of the Dragon and younger sibling to Aka, Manmoth, Gothano, Chi-You, the Rat King, the Toad Baron and Jagwar.
Kitsune entered the history of the Foot Clan when she rescued Takeshi Tatsuo and brought him to her home, where a dose of the Utrom primordial ooze regrew his severed leg. She remained at his side for many years afterwards, and continued giving him regular doses of the ooze in order to reverse the aging process. However, she did not foresee Oroku Maji and Masato staging a coup against Tatsuo when he was at his weakest. The founder of the Foot Clan was killed, and Kitsune cursed Maji and fled.
She returned to the Foot Clan many years later when Maji's son Saki - who was also the reincarnation of Takeshi Tatsuo - encountered her and learned of the events of his past incarnation. She became Saki's lover and the mother of his sons as well as his greatest asset, and eventually she concocted a method of making him functionally immortal. At some point she went into a form of hibernation to await his return.
Her first important act upon reawakening was to brainwash Leonardo with her dark magic, twisting his memories and self-doubts, and causing him to believe Splinter had abandoned him and that his brothers were dead. Her brainwashing was incomplete due to the connections he still had to his family, but it was enough to turn him against them.
She was present when the Foot Clan and the Hamato family engaged in the Gauntlet, an ancient ritual that pitted the Turtles against their mutant foes and Shredder against Splinter. Shredder was mortally wounded during their duel, and one of his last statements before he committed seppuku was that he would find a way back to Kitsune.
With Splinter taking control of the Foot Clan, Kitsune was kept close by so that he could keep an eye on her. She remained in her own room most of the time.
She also had been brainwashing the mutant fox Alopex for a long period of time, in a subtler form than Leonardo's brainwashing. This came to a head when she tried to reclaim the body of Oroku Saki, using Alopex to fight alongside her and ensnaring the minds of some of the Turtles. She successfully made her way from the Foot Clan with Shredder's corpse, but discovered that his severed head was still in Splinter's possession.
She reappeared at the Pantheon Family Reunion, where she revealed the ultimate goal of her relationship with Shredder: by turning him into the immortal Dragon Warrior, she was planning to groom him as a possible vessel for the resurrection of the Pantheon's father, the Dragon. Furthermore, she hoped all of humanity will be annihilated in the process.
In Japanese, kitsune (狐) means "fox," and in Japanese mythology they are seen as cunning shape-shifters and magicians, often taking the form of a beautiful-looking woman.