I wanted to expand on Chelsea's earlier comment that addressed Lady Brett Ashley's lack of personhood in the novel. This is my first experience with The Sun Also Rises, so this could be a bit out of left field, but I considered her more as a symbol than a character. It is interesting to consider Lady Ashely as the War, personified. The novel begins with a rather understated introduction to Brett, as she kind of just appears in the dance club in Chapter 3. Then, as noted by a number of characters throughout Book I, Lady Ashley is defined as being a classy lady, just as WWI came about somewhat randomly and was initially recognized as a positive venture. As the novel continues, Lady Ashley affects (or, rather, infects) the minds of the men around her. As Chelsea noted in her post, "Brett is a character denied the same sorts of agency that Jake, Bill, Robert, and Michael have access to." Rather than operating as a character with her own agency and mobility, Brett is the ever-present figure and topic that these men discuss while the War and its pain are the undertone. 

What this portrayal of Lady Brett Ashely as a symbol rather than a character says about Hemingway is another topic entirely, but understanding her in this way gave the novel a nuance that I was willing to entertain.