On the surface, Marjorie Harvey is the poster child for the new era that Fitzgerald loved to depict; she's relentlessly modern, trendy, and socially savvy. She's the Queen Bee of her town's youthful population – the Regina George, the Blair Waldorf, the Cher Horowitz of her time. Marjorie is the be all and end all of what guys want and what girls want to be; she's well-dressed, gorgeous, witty, and incredibly popular, even on a national scale (she's practically a superstar at the then-all-male Ivy League colleges she visits for parties and football games). Marjorie is truly a thing to be reckoned with.
The only thing that could possibly do any reckoning with Marjorie would have to be a force equal to Marjorie herself – which, oddly enough, turns out to be the underdog, Bernice. By fashioning her cousin in her own image, Marjorie creates the agent of her own downfall; in Bernice, she takes on a project that succeeds all too well. The new Bernice, as constructed by her cousin, has all of the traits that make Marjorie successful: beauty, wit, style, and ultimately, true ruthlessness.
However, Bernice has something Marjorie doesn't possess (and probably never will) – honesty and authenticity. Marjorie is kind of a major hypocrite, for though she claims to be as modern and contemporary as can be, she also upholds some elements of the traditional definition of women. While she has no qualms about bobbing Bernice's hair, she makes no mention of ever bobbing her own flowing, princess-like, golden locks. And likewise, while she proudly declares herself a fun-loving, hedonistic "gardenia girl," she eventually gives in to that old demon, jealousy.
Marjorie's possessiveness of Warren demonstrates her hidden connection to an older vision of womanhood; surely, if she were really wholly invested in living in the moment and not worrying about the future, she wouldn't care about losing her most faithful suitor (who she doesn't even love!) to her cousin. To make things even worse, she's something of a misogynist – she doesn't have any real female friends because she arrogantly thinks that all girls (with the exception of herself) are stupid. To that, we have one response: Seriously? Come on, Marjorie – what kind of liberated modern woman are you?
In conclusion, Marjorie is nothing but trouble, and when Bernice takes her revenge at the end, we just want to giggle wildly along with her.