Jack Duane is the first educated man Jurgis has ever met personally. The two guys meet in jail twice: first, when Jurgis assaults Connor, and second, when Jurgis gets ten days for beating up a bartender (who stole a hundred dollar bill from Jurgis, but of course, no one believes him). Anyway, Jack Duane is an East Coast college-educated engineer from a good family. He falls into a life of crime because (a) his dad killed himself after a financial deal went bad, leaving Jack to look after his mother and sister, and (b), Jack invented some kind of important communications device that gets stolen by a large corporation. He spends all of the family's money in a failed effort to sue this large corporation for the patent, leaving his family poor. So Jack Duane skips town and becomes a safecracker. Having been screwed over by society, Jack Duane decides to stop respecting society's rules, and Jurgis follows his example.
Jack Duane offers to help Jurgis when Jurgis gets out of jail the first time, but Jurgis is not ready to start on his life of crime. He still has his duties to dying Ona and to Baby Antanas. But after Jurgis's second jail sentence, he is fed up with the world. So, heck yeah, he joins Jack Duane. The two guys start mugging people on the Chicago streets. Jurgis feels vaguely guilty about leaving their marks unconscious on the Chicago streets in winter. He hears that one of their victims loses three of his fingers to frostbite, much like poor Stanislovas. Jack Duane explains, though, that it's either us or them. To save yourself from being a victim, you have to victimize other people. Anyway, no one in this rough and tumble world is innocent. So no one really deserves to be protected.
Jack Duane gets caught red-handed breaking into a safe once he and Jurgis start their life of crime together. So he leaves Chicago on bail. By this time, Jurgis is deep enough into Chicago's criminal world that he can manage without Jack Duane's assistance. There are a couple of things that are interesting about Jack: first, it's worth noting that he is one of the only people who does Jurgis any kind of favor. Because Jack Duane is outside of society and its rules, he actually seems less competitive (and evil) than "legitimate" businessmen like Mike Scully or Connor. Also, Jack Duane goes to show that it isn't just poor immigrants who are victims of large corporations' greed. Middle-class gentlemen can also fall into trouble with these guys. Sinclair is saying that, yes, Jurgis has it worst, but we are all caught up in a system that is stacked against ordinary men and women.