Who is Jordan Baker, and why is she staying with Tom and Daisy?

Jordan Baker is visiting Daisy. She is Daisy's longtime friend, and a professional golfer who cheated in order to win her first tournament. Jordan is extremely cynical, with a masculine, icy demeanor that Nick initially finds compelling. The two become briefly involved, but Jordan rejects him on the grounds that he is as corrupt and decadent as she is.

Jordan was staying with Tom and Daisy because she was playing in a golf tournament.

‘Jordan’s going to play in the tournament tomorrow,’ explained Daisy, ‘over at Westchester.’