He was portrayed by John G. Connolly.
Attacking the XXX Agency
He was a member of the Unit as Darius Stone (the new xXx agent and protagonist of the film). When Stone started the riot, Cobb stayed loyal to Deckert.
He is seen helping Deckert to plot a coup against President James Sanford in order for Deckert to become the new president and blame the event on NSA agent Augustus Gibbons, whom they took prisoner earlier. Deckert also arranged for Cobb to head to Bora Bora to assassinate Xander Cage (the original xXx agent), though Xander faked his death to live in self-exile at the Dominician Republic. When Stone boards the "USS Independence“ to gain information about Deckert's plans, Charlie Mayweather activates the alarm, which makes anyone on-board the ship, including Cobb, aware of Stone's presence. During Stone's escape, they mock each other through a walkie talkie. Cobb arranges for Liebo to commandeer a tank to kill Stone, though Stone escapes after killing Liebo, much to Cobb's anger.
Final Fight and Death
During the ambush on the White House, Cobb and his men kill the guards of President Sanford. However, Stone, NSA agent Kyle Steele, and a gang led by Stone's old friend Zeke were able to formulate a successful plot to fire at the White House with a hijacked tank, allowing Stone and Steele to free Gibbons before killing Charlie and the rest of Cobb's men. Despite the setback, Deckert and Cobb flee on-board the escape train by taking Sanford as hostage.
When Stone boards the train, Deckert sends out Cobb to deal with him. Cobb ambushes him in the train's kitchen by hiding in a shelf and stabbing Stone in the arm, pinning him to the wall. During the fight, Cobb drops his lighter as he attacks Stone with a meat cleaver he picked up before, unintentionally cutting a gas pipe of the oven. When Cobb pins Stone down by pressing a shelf on him, Stone ignites the lighter and throws it into the gas, causing a giant explosion which finally kills Cobb, blasting his burning corpse out of the train window at full speed.