From The Paris Review:
Damned good-looking, it is.
Imagine Hemingway, the great economist of words, deciding just how he would introduce perhaps his most enduring siren. Original drafts of the novel open with the character Ashley (better known as Brett), though she would eventually come to play a smaller role. Hemingway was bewitched, at the time of writing, by the self-possession of the real-life Lady Duff Twysden, and she—rather than his wife, Hadley—would serve as the partial inspiration for The Sun Also Rises’s heroine. (Indeed, he would dedicate later editions of the novel to her.) Poor Hadley would be left out again when her husband took up with Lady Brett’s other progenitor, Pauline Pfeiffer, who in 1926 came to France to assist Mainbocher at Vogue. In addition to being a fashion writer, Pfeiffer was an accomplished journalist, an intellectual who fit easily into Hemingway’s Paris crew. It seems Hemingway’s rigid conception of a professional, globe-trotting man’s man—a fan of hunting, boxing and bullfighting—shouldn’t settle for pretty; he’d want damn good-looking. “Damn good-looking”—Hemingway’s highest female accolade—is also, in the form of Lady Brett, damn witty, damn intelligent, and damn good in bed.
More here.