The Teenage Mutant Ninja Turtles are famous for being a quartet of superheroes, but the group has expanded in recent years to include another member.

Comic creators Kevin Eastman and Peter Laird's beloved characters Leonardo, Raphael, Donatello, and Michelangelo may be the most recognized lineup of the Teenage Mutant Ninja Turtles there is, but their ranks have since expanded to include a fifth turtle. Jennika, a former member of the Turtles rival ninja group the Foot Clan, would become the asset the group never knew they needed. After an encounter with the former co-leader of the Foot Clan, Karai, the formerly human Jennika is mortally wounded. Jennika's injury proves severe leaving an emergency mutagenic blood transfusion, delivered by the brains of the Turtles, Donatello, as the former human's only chance of survival.

Introduced into the series in 2015, Jennika (or Jenny as she's commonly referred to) arrived on the scene as a loyal member of the Foot Clan, who were now under the command of their former ninja turtle adversaries led by the anthropomorphic rat sensei/father Master Splinter. It would not be long before Jennika would turn against her new benevolent employers and partner with the New York criminal tech group known as The Street Phantoms in an attempt to return the clan to its former "glory". However, Jenny would reach an understanding with Splinter, remaining on the team and even developing a romantic relationship with the Turtles' longtime ally Casey Jones.

Unlike her four turtle compatriots, Jennika began as a lethal assassin first and foremost, initially showing little compassion and pity for her enemies. Jennika's newfound transformation, as a result of blood given by Leonardo, not only alters Jennika's physical appearance but improves her ability in working with an inherently good-natured team. To fit in with the other turtles, Jennika is often represented by her own yellow bandana and a pair of metal claws from her time as part of the Foot Clan.

While Jennika is the first official fifth member of the Teenage Mutant Ninja Turtles in the comics continuity, there have been quite a few fifth members in other forms of TMNT media. The third season of 1987's Teenage Mutant Ninja Turtles series featured an episode entitled The Fifth Turtle, centered on a young fan of the titular group named Zach who would eventually become a reserved 5th member for the group. There would even be a show dedicated to a fifth element of the group of ninjas in the following decade, with the short-lived Teenage Mutant Ninja Turtles: The Next Mutation.  The fifth turtle of that series would be Venus, another ordinary turtle exposed to mutagen along with the four originals but forgotten by Splinter.

Though not originally a turtle, Jennika's learned to cope with her new form and has cemented herself as not only a fan-favorite hero but an integral member of the Teenage Mutant Ninja Turtles.

Next: TMNT: Why the Turtles Reboot Needs To Parody Superhero Movies