Jencko is the leader of the Turks.
He first appeared in TMNT 2003 season 4 episode 11, "Still Nobody", where he was beating a former member with a pipe until he spotted an old man and prepares to chase him. Luckily, a police car is in the area and the thug disappears into the shadows. An ambulance arrives and takes the seriously injured youth to the hospital — fortunately the old man interrupted in time to save the kid's life. As the elderly gentleman gives his statement to the police, Jencko is seen listening in from a hiding spot.
X-Ray, Nobody's informant, is making his way along the sidewalks of the city when a car full of gigantic Turks members pull up next to him in a car. The thugs tell X-Ray that Jencko wants to talk to him.
At the abandoned club that the Turks use as their headquarters, Jencko was threatening X-Ray for the gang leader knows that it's X-Ray that's been feeding info to Nobody — and thus X-Ray is in big trouble. Before Jencko can beat X-Ray with his pipe, Leonardo drives the Battle Shell through the front doors and into the club, with Raphael zooming in on his bike. The heroes attack the gangsters as they open fire with their laser weapons. Jencko takes X-Ray hostage. Leo and Raph defeat various thugs while Nobody tracks Jencko. The costumed hero manages to use his grappling apparatus to pull X-Ray to safety.
With his ally out of danger, Nobody focuses his efforts on stopping Jencko and knocks the hoodlum to the ground with a solid punch to the ribs. The other Turks mill around, but they don't seem to want to fight. They're afraid of Jencko and don't know what to do. Nobody tells them that they can walk away from this life of violence and have a shot at a better life (rather than doing time in jail or worse). This speech works and the gang members drop their Turks jackets, but Jencko suddenly pulls out a grenade launcher and starts firing. Leo and Raph get the other kids out as Jencko fires grenades in all directions. Nobody knocks him out, but the place is about to go up in flames from the explosions. Nobody grabs Jencko and bails just in time. The fire department and police department arrive. The flames are doused and Jencko is arrested by the police.
Later at the end of the episode, it was discovered that Hun and the Purple Dragons have been the ones selling the weapons to Jencko. After seeing Jencko's arrest on the news while being driven in his limousine to the grand opening of the Oroku Saki Memorial Library, Hun calls up Vang telling him to scratch Jencko and the Turks off his list of possible recruits and to see if they can salvage the weapons they sold to him.