Jake Barnes is not merely the narrator (storyteller) of The Sun Also Rises. He is also its protagonist, or main character. That means that the novel is driven by his needs and desires more than those of the other characters. Jake's main need, of course, is for Brett. He wants to love Brett and to be loved by her in turn. The bitter irony of The Sun Also Rises: Although Brett is more than willing, Jake's sexual attraction can never be satisfied, because he has been castrated in combat during World War I. Because he feels sexually drawn to Brett, who is attracted to him in turn, Jake's inability to consummate their mutual desire makes being near Brett or even thinking about her sheer agony for him.
Like Brett herself, as well as her fiancé, Mike Campbell, and the Count Mippipopolous, Jake is a casualty of war. He tries to heal himself, at least emotionally, with friendship, food, and fishing. (Fishing is almost sacramental for Jake; notice that he and Bill drink less when they are in Burguete.) Aficion (passionate expertise, especially regarding bullfighting) provides Jake with comfort, because it offers him a measure of control over a world that otherwise frightens him by virtue of its extreme randomness. And of course, like Brett and Mike, he dulls his pain with alcohol, quantities of drink that are almost incomprehensible.
Jake never sees the big picture, just an unending stream of details. He lives in the present, refusing to analyze things. (Notice that Hemingway refuses to provide us with background on his protagonist's youth, aside from one brief memory of an afternoon in the American Midwest. As mentioned earlier, there are no combat scenes in this novel about war and its effects.) This is not, however, because Jake is a superficial person. In fact, he is capable of the most penetrating insights, as when he says of Brett, "I suppose she only wanted what she couldn't have." Rather, it is due to the fact that if Jake were to examine the reality of his situation for even a moment, that reality would probably result in his suicide.
Despite the crushing disappointment that is his life, post-war, Jake tries to behave well, in a moral sense. He fails in this when he sets Brett up with the bullfighter Pedro Romero, thus hurting her admirer (and Jake's friend) Robert Cohn and running the risk of destroying Romero's career before it has even begun. In fact, Jake wants very much to damage Cohn. He can't stand that someone whom he feels to be deeply unworthy of Brett's love should have her — and Cohn has never served in combat. Thus, he is less than a man in Jake's estimation. Notice that Jake approves of the Count, whose body is tattooed with war wounds. Though Romero is not a veteran per se, he faces death every day in the bullring. Therefore, Jake sees Romero as a satisfactory proxy for Jake himself; as a result, he does indeed "pimp" for Brett, just as Cohn says.
Jake fails morally during the fiesta of San Fermin, and he knows it. Still, like all of Hemingway's heroes, he stoically tries to get on with life anyway. Note that despite his horrifying physical condition, Jake never pities himself, except on occasion when he's very drunk and — significantly — alone. (A Hemingway hero would never bellyache about his problems within earshot of someone else.) And although Jake cannot have sexual intercourse, he undeniably can love others: his friend Bill Gorton, his mentor Montoya, and of course Brett herself. Jake's devotion to Brett knows no bounds, as proven by the novel's final chapter, in which he travels cross-country to be with her in Brett's time of need. Though Jake thinks of himself as someone for whom love is impossible, precisely the opposite proves true.