Rosalie is a member of the Cullen clan, and one of Edward's adoptive siblings. According to Bella, Rosalie has "a beautiful figure, the kind you saw on the cover of the Sports Illustrated swimsuit issue" (1.110), and later Bella describes her as "beyond belief" (Epilogue.34). Though Carlisle originally intended for Rosalie to be Edward's companion, it's very clear that neither are romantically interested in each other. Rosalie is now with Emmett, and every couple of years they get married again.
As you may have noticed, Rosalie is by far the coldest Cullen to Bella, and she remains that way through the very end of the novel. She never opens her arms to Bella like the rest of Edward's family does. She gives Bella nasty looks at every opportunity and literally runs away anytime Bella joins a conversation. We learn why she acts this way from Edward, who explains that Rosalie struggles the most with their vampirism, and she's jealous of Bella, since Bella is a human.
It's important to note, that Rosalie doesn't let her issues with Bella overshadow her duty to her brother Edward. When James enters and starts to cause trouble, Rosalie camps out at Charlie's house to make sure that James's female vampire companion, Victoria, doesn't try anything.