Though he is the first character to appear in The Sun Also Rises, Robert Cohn is not the novel's hero; rather, Cohn is the hero's foil, the character who will serve to highlight the protagonist's strengths and weaknesses by contrast. According to Jake, at least, Cohn is insecure and self-conscious. He is perpetually broke and a dabbler in the arts. Cohn is voluble and naive. (A kind of overgrown child, he even bursts into tears more than once.) And Cohn allows himself to be controlled by the women in his life: his mother, his wife, and his lover. Later, Brett reduces him to a pathetic puppy. Most significantly of all, Cohn is not a veteran of the Great War. At least initially, Cohn appears to be everything Jake is not, and vice versa.
In fact, if there is a typical Hemingway hero, Robert Cohn is something like the exact opposite of that. He learns not by doing, but by reading. (Ironically, the world-famous writer Hemingway was suspicious of books. A fundamental tenet of the author's credo was: Believe only what you have seen with your own eyes.) Most of Cohn's ideas about life and how to life it appear to have come from the printed page. He wants to visit South America because he has read about it. Most likely, his badly-misplaced fantasies of a lasting love affair with Brett come from books, too. Cohn believes in true love and can't conceive of sex as recreation, as Brett does. No wonder their brief time together ends disastrously for him.
Robert Cohn is a boxer, a practitioner of one of the so-called "blood sports" (like bullfighting) admired by Hemingway, and by Jake. Cohn fights effectively, too; note how he dispatches Jake himself as well as Mike with his fists and only fails to beat Romero, who possesses a sort of superhuman resilience. And yet, Jake appears not to admire Cohn for his pugilistic skills. Somehow Cohn lacks a sufficiently nuanced understanding and appreciation of the art of boxing. This may say more about Jake himself and his competitive nature than it says about Cohn, but again, Cohn's function in The Sun Also Rises is to illustrate the protagonist's qualities — good and bad — by comparison. In fact, it may be precisely Cohn's resemblance to Jake (both are writers and athletes, and both attract Brett) that the latter finds so unsettling. Cohn is a living reminder to Jake of what he used to be, before the war ruined everything. Ultimately, Jake wants to hurt Cohn so that Cohn will no longer be unwounded. Though admirable in many ways, Hemingway's hero has a dark side.