He serves as one the two main antagonists of the season (other being Sh'Okanabo). He is also is a villain in the IDW Comics version but unrelated to the Jones family.
Darius Dunn first appeared the second episode of TMNT: Fast Forward, Obsolete when he came to Cody's penthouse after he was told by Constable Biggles about how Cody was outside the penthouse the day before. He lectures Cody about how he must stay in the penthouse, claiming that it is for his own protection, as there are people who will hurt since he is to the O'Neil Tech fortune, which is supported by Sterling, Cody's robotic butler, and caretaker.
However, due to the TV show's cancelation, it's unknown what happened to Darius. It's possible that he got arrested for his crimes.
Darrius Dun appears in the IDW comics as a villain alongside the Street Phantoms. In this version he is a crime boss and former athlete who tried to gain control of the New York underworld through the technology his subordinate Libby Meitner had created and tried to recruit her ex-husband Harold Lillja, an ally to the Ninja Turtles to his cause. After his forces were defeated by the Foot led by Splinter and the Turtles, and he was defeated Splinter in battle, his refusal to surrender caused Splinter to order his Foot subordinate Jennika to execute Dun (an act that would cause Leonardo, Donatello, and Raphael to join Michelangelo in breaking ties with the Foot). Dun later returns as a spirit. He is also unrelated to the Jones family.