Jake Barnes is the novel's main character and its most straightforward representative of the lost generation. Barnes has been literally wounded by the war. His injury is never explained entirely, but it is clear that it prevents him from intimacy. Barnes' character represents the psychological trauma of this generation because he appears entirely intact and unscathed to most people. The permanence of this damage symbolizes the inability for people to entirely recover from war. The fact that Barnes' wounds interfere with reproduction speaks to one of the main ways his generation is "lost": it is impossible to simply move on and continue living. Similarly, traditional values and hopes that once guided life, such as romantic love and family, appear quaint, naive and out of reach.
If Jake Barnes is representative of the men of the lost generation, Lady Brett Ashley is representative of the women. She loves Jake, so his alienation from traditional values affects her. Ideals of femininity that other generations of women trusted to bring them love and family are of no use to Brett, who has decided to discard them. She is cynical about romance and never pretends to be monogamous. With her short hair and short skirts, Brett does not conform to gender norms -- she is the quintessential androgynous flapper.
Hemingway uses the character of Robert Cohn to introduce traditional ideals into the text of the novel. Cohn, unlike Jake Barnes and Lady Brett Ashley, has not become disillusioned. He continues to put stock in romantic love and traditional gender roles and his ideas of heroism and honor make him ridiculous in the eyes of other characters. Cohn is the least popular member of the group, frequently an object of scorn or pity. His status among the other characters shows how ridiculous and sentimental old-fashioned romanticism seems to the lost generation.
Pedro Romero is not a member of the lost generation but he is a crucial part of its portrayal. Like Cohn, he serves as contrast to the other characters. As a bullfighter, he participates in a very traditional, historic ritual. His advice that Brett should grow her hair out shows his old-fashioned attitudes about gender roles. While Cohn annoys other characters with his romantic sensibilities, Romero impresses them. One reason for this is that he is innocent, not foolish. As a 19-year-old Spaniard, he missed the worst of the war and thus provides a glimpse of what the lost generation might have been. The fact that Brett falls in love with him shows that under their careless and cynical exteriors, members of the lost generation mourn what the war cost them.