Jordan Baker is Daisy Buchanan's long-time friend with "autumn-leaf yellow" hair, a firm athletic body, and an aloof attitude. She is Nick Carraway's romantic interest and girlfriend for most of the novel and a professional golfer with a slightly shady reputation and a penchant for untruthfulness.
Jordan is a golfer—a professional golfer. Already, we know she's different from Daisy. Where Daisy is always fluttering and babbling and giggling and basically acting like a dumb girl (her words, not ours), Jordan is hard, direct, and cynical. And she's bored to tears. We don't know much about her family, except that she has "one aunt about a thousand years old" (1.137), but we know that she and Daisy spent their "white girlhoods" together (1.140). Given the looks that Daisy and Tom give each other, we suspect that she might not be so "white" (as in, pure) anymore.
But there's a problem with her game. She's a cheat. And Nick describes her as not just a golf cheat, but a cheat at life. Ironically, she and Nick form an attachment in the novel before ending their romantic relationship at the end of the novel.