Some TMNT stuff really isn't for little kids.
Jennika is a young Foot Assassin, and a loyalist to the Foot Clan rather than to The Shredder. As a young girl, she was driven from her home when she found her father beating her mother, and lived on the streets afterwards. She took up with a young man who pulled her into a life of petty robbery, which ultimately led to her imprisonment as an accessory to murder.
Her first encounter with the Foot took place in prison, where an imprisoned Foot ninja defended her from other convicts and became her fast friend. The woman trained Jennika in secret as they plotted to escape the prison, but Jennika's friend was killed by another inmate just as they had gained their freedom. She found her way to the Foot and became one of their number, using her newfound skills at killing on the woman who had murdered her friend.
After Shredder's death, she distrusted the new leader following his death and questioned why this "rat" was fit to lead. She was convinced that he was weakening the Foot Clan with his unwillingness to kill without hesitation, and saw his merciful actions as signs of cowardice and weakness, believing that quick and deadly action was necessary to remain supreme.
As a result, she secretly associated herself with Darius Dun and the Street Phantoms. When the Phantoms attacked the Foot Clan, Jennika attempted to kill Splinter in single combat, but was defeated quickly.
After Splinter humbled her, she began to realize the error of her ways, and pledged her service to the Foot's new leader and his sons. Splinter believed in her change of heart, though he still had to penalize her for her act of disloyalty. After being impressed by her faithful service, he assigned her to guard the tomb of the Shredder, which she did faithfully until she was attacked by Kitsune and Alopex.
During the Triceraton invasion, it was confirmed that Splinter had selected her as his new chunin. However, she was still very impulsive and required his guidance. She was also deeply conflicted about Splinter's plans to kill Commander Zom, eventually arranging for the Turtles' escape from the Foot Clan headquarters in order to stop him. She was not penalized for her disobedience after the invasion ended, and was instead sent out by Splinter to protect Casey Jones.
After the Turtles brought a number of orphaned children to the Foot Clan, Jennika helped care for them.
When Karai's attempt to reclaim leadership of the Foot failed, she reached out to Jennika for a truce. Karai attempted to sway the chunin's loyalties away from Splinter, but Jennika refused, and Karai stabbed her.