Nick might end up "halfway in love" with Jordan, but he consistently describes her as cynical, having seen too much and heard too much to be fooled by anybody. And perhaps because of her dishonesty, she is aware more than anybody else in the book that appearances are deceiving.
Jordan is possibly the least important of all the major characters in the book, yet she provides an important contrast to Daisy Buchanan. She appears first in the Buchanan’s home, a young woman with too much time on her hands. In some ways, she epitomizes the concept of "ennui" – she is bored to tears, except for her active sports career in golf. Cynical and hard, she cheated to win her first golf tournament. This in itself is evidence of her practicality. Ultimately, she and Nick end up "together" (in a fashion) and Nick mentions how grateful he is that she is not like Daisy. That is, she is not the kind of girl who holds onto the past, a girl "too wise ever to carry well-forgotten dreams from age to age."
But Daisy and Jordan aren’t quite polar opposites after all, and it is Jordan’s place in that "rotten crowd" that drives a wedge between her and Nick. Well, maybe. We offer a different explanation in Nick’s analysis, so take your pick. But you can’t really argue about the fact that Jordan is just as careless as Daisy. Daisy "smash[es] things up" and then "retreat[s] […] into […] vast carelessness." Does that last word look familiar? Nick tells Jordan she is "careless," and after a brief attempt at denying it, she pretty much gives him a "whatever" in return. She callously says that other people have to stay out of her way. This might be the reason Nick comes to dislike her, but the reason he is taken in at first is her following line: "I hate careless people." Looks like they have something in common, after all.