What's most remarkable about Brett, Jake Barnes's love interest in The Sun Also Rises, is her utter modernity. In her book Terrible Honesty, the writer Ann Douglas points out that the 1920s is the earliest decade that seems modern or contemporary to us. Hemingway's Brett is proof of that. She is not the least bit old-fashioned. If she were made real and somehow transported to high-society Paris, London, or New York of the present day, she would fit right in.
Brett parties hard. She is unapologetically sexual and aggressively promiscuous. She even wears her hair cut short, like a man. She's one of the boys (she refers to everyone she knows as "chaps"), whether the boys are the group of gay men in whose company Brett is first seen, or Jake, Cohn, Mike, Romero, and the Count, all of whom she has attempted affairs with. And yet she strikes all those who meet her, even Bill Gorton, as attractively feminine. Notably, Brett seems not to have any female friends; she is a "man's woman." As a result, nearly all the men in the book fall in love with her: not just Jake and Cohn, but Mike, Romero, the Count, and even the drummer in the Paris nightclub and the Basque peasants who see her on the streets of Pamplona. Jake and Mike love Brett so passionately, in fact, that they are willing to allow (and in Jake's case, encourage) the object of their affection to pursue affairs with other men.
Yet Brett's breezy style and the way in which she is game for any experience (remember that she doesn't flinch at the carnage in the bullring) hardly indicate personal happiness, much less inner peace. She often tells Jake that she's miserable. Also, Brett seems to feel dirty, as evinced by the number of baths she takes during the story; indeed, she may be attracted to Jake partly because he is "clean" — that is, asexual. Romero may be "clean" in another way, a virgin. After all, he is only a teenager, in a very conservative society.
Brett is unhappy for three reasons. First, like Jake, Mike, and the Count, she is a war veteran. Though she of course did not see combat, Brett served in a military hospital, an experience that was surely harrowing just the same, especially considering the newly-brutal weapons employed in World War I. Second, there is no place in her society for a woman like Brett — a female Don Juan, if you will. (This may partly explain her lack of relationships with women. She has been ostracized by her female peers for her brazen sexuality.) Finally, Brett is nearly as tormented by their unrequited love as Jake is. In fact, her serial affairs can be seen as attempts to fill the void created in her by Jake's unavailability. Notice that immediately after Jake tells Brett he loves her, she says she is in love with Romero, as if to bury her powerful, reciprocal feelings for Jake.
Although Jake Barnes is the protagonist of The Sun Also Rises, Brett serves as the novel's center, its objective focus. She is the "sun" around which the other characters orbit, starstruck, in the way that the Basque peasants place her on a wine cask and dance in adoration during the fiesta, as if Brett were a goddess. Brett cannot relate to Jake's Catholicism partly because she herself is an object of worship and dislikes sharing the altar with other deities. This does not mean that Brett is selfish, however, or narcissistic. She is merely realistic about, and accepting of, the power she has over men.
Unlike Jake, who is in the same boat at the book's conclusion as he was on the first page, Brett has changed by the end of The Sun Also Rises. First, she has grown truly capable of loving someone besides Jake — an important step if she is to live a life less than utterly miserable. Even more striking: Though she loves Pedro Romero — loves him madly, in fact — she ends their relationship because she knows to continue it would harm Romero's career. Brett has demonstrated a capacity for generosity that was not apparent at the start of the novel, a moral strength we could not have imagined her possessing. Now if she can only grow strong enough to leave Jake — to leave him in peace.