Jack Duane is the closest thing Jurgis has to a friend in the entire novel. It is quite appropriate that Jurgis meets Duane in jail, for this is an ironic statement: Jurgis has to go to jail in order to become a criminal. Outside of prison, Jurgis is an honest man. Only after he is sentenced for his "crimes" does Jurgis go to a place where he learns how to break the law.
A minor character, Duane is nonetheless significant. In order for the thematic nature of The Jungle to be evident, Jurgis has to explore all options available to him. Joining the underworld of Chicago crime enables Jurgis to experience economic success, and eventually realize the price of success. When Duane is no longer necessary, he is conveniently sacrificed — both by the mob and by Upton Sinclair.