Robert Cohn is the first character we meet in the novel, even before we get to know Jake. We immediately find out everything we could possibly want to know about Cohn – we learn about his family, his school years, his failed marriage, and his half-hearted ambitions to become a successful writer. But what does this accomplish? One would usually reserve this sort of treatment for the novel’s hero – but Cohn is certainly not our hero. If anything, he’s the opposite of what an ideal Hemingway hero should be (strong, opinionated, knowledgeable, and brave, among other things – in other words, like Jake, minus the whole impotence thing). However, Robert Cohn is the character that sparks the whole plot of the book. It’s his affair with Brett and its fallout that sets off the whole chain of jealous explosions that pepper the novel and fuel its action. While he’s not the hero, or even a particularly sympathetic character, he is central to the novel’s plot.
Jake comments early on that Cohn brings out the worst in people – and lo and behold, he consistently does. Discussion about Cohn brings out nasty dark sides to characters that we are sympathetic towards, like Jake and Bill, who both show alarming anti-Semitic tendencies in their put-downs of Cohn. Cohn is also marked as different from Jake, Bill, and Mike in a very significant way – he alone of all these men didn’t fight in the war. He has a kind of curiosity about the war, but still hangs on to outdated notions of love, chivalry, and battle that none of the other men believe in. This clash of values is one of the major sources of conflict between the male characters. In all of these ways, Cohn serves more as a catalyst than an actor, since he provokes situations, but doesn’t follow through with action.