Judy Jones is not Dexter's antagonist in the sense that he makes an enemy out of her. She does pretty much cause all his problems, though: his one-sided desire is the big hang-up in "Winter Dreams." Throughout the story, she remains the One Who Got Away. She obviously has some affection for him (though never enough to overcome her own self-absorption).
Beyond her actual feelings, Judy also comes to stand in for all the things Dexter wants out of life: upper class status, beauty, romance. By the end of the story, Dexter has realized that these dreams of Judy and of romance are never going to come true, no matter how much money he makes. In a sense, Dexter loses to his antagonist. He never actually grasps those dreams that Judy comes to symbolize for him.