Brett is not a Brat.

Many of my peers, mostly male just to point out, believe that Brett is a terrible character who can be characterized by another five lettered word that starts with a "b." I disagree, quite strongly. Primarily, there is the problem with the narration through Jake's perspective. No matter how stoic a character he may seem on the surface, he does subtly inject self-sympathy into the story. Jake portrays Brett as a complex, unattainable love interest who plays around and has a lonely life. He loves her, there's no doubt about that, but he's forcing his love for her to take precedence over her love for him. In other words, the readers can see and coo over how much Jake thinks of Brett, but cannot gain access to her thoughts. To them, Brett is a party girl with a sad history and promiscuity as wide and as deep as the sea. In a way, they are correct because she does taken on those characteristics, but on the other hand, they are absolutely wrong.

When I analyze Brett, I see her as a tragic figure. Her true love - one that must have been something to ensnare her - died in the war, and her next love (assuming her first true love wasn't Jake's masculinity because that would just be shallow) cannot provide her with the comfort she needs. She marries the first man, an abusive tyrant, who comes along and suffers terribly. And despite her great looks and lovely personality, she has an inferiority complex. She drinks to cover it up, sleeps around to hide it, and cannot be with Jake absolutely because of it. For Brett, I think, Jake is the perfect man, one who cannot taint a relationship with sex and whatnots. While she is drawn to that idea (as we all are attracted to something ideal), she is too insecure of herself to jump for it. Instead, she finds temporary solace in the flawed guys such as Mike and Robert.

Perhaps my analysis is bias and skewed. That's your opinion. But everyone has their faults, some more obviously than the other. If you are willing to forgive Jake for his inability to communicate his love and need, then you ought to consider pardoning Brett for her insecurities.

Jake as Septimus

I didn't believe that a book could be more sorrowful than Mrs. Dalloway until I read The Sun Also Rises. God, the latter was just like a punch to the gut. Jake Barnes, an incredibly normal and respectable guy, is so very much like Septimus Smith. There is something fundamentally lacking about Jake and this wound, physical and physiological, prevents him from seeking the love that he so obviously desires. And yet everywhere, he is bombarded with other people in love. Walking in the street, he notices "a man and a girl... walking with their arms around each other (83)." He silently criticizes the careless matrimonial behavior of Robert Cohn, and derides the crowd of chaps that follow Brett around who are so thoughtless with their gift to procreate life. 

Septimus too suffered from his inability to feel particularly because everyone around him showered him with feelings. To realize that you lack something so essentially ingrained in everyone else is a frightening realization. It is an unbelievably lonely feeling. To escape it, Septimus commits suicide and Jake exhibits bitter behavior. What Jake will do though remains to be seen.