Judy and Irene act as foils in “Winter Dreams,” meaning that when contrasting the characters with each other, readers gain insight into each of the characters (and the story as a whole). In particular, Fitzgerald intentionally sets Judy up to be the fickle femme fatale in Dexter’s life and Irene to be the stable and reliable romantic partner.
In the following passage, the narrator allows readers a peek inside Dexter’s mind as he compares his current relationship with Irene to his previous relationship with Judy (though he doesn’t mention her by name):
He knew that Irene would be no more than a curtain spread behind him, a hand moving among gleaming tea-cups, a voice calling to children... fire and loveliness were gone, the magic of nights and the wonder of the varying hours and seasons... slender lips, down-turning, dropping to his lips and bearing him up into a heaven of eyes.... The thing was deep in him. He was too strong and alive for it to die lightly.
Here, Dexter metaphorically describes Irene as “a curtain spread behind him,” “a hand moving among gleaming tea-cups,” and “a voice calling to children,” three comparisons that communicate how supportive, proper, and maternal she is. While none of these are overtly negative descriptions, it becomes clear when Dexter thinks back to his times with Judy that Irene lacks passion and sensuality. Unlike Irene, Judy offers him “fire and loveliness” and “slender lips […] dropping to his lips.” In this way, the women act as foils for each other, illuminating what the other lacks (or doesn't).
Not long after this passage in the story, Dexter decides to leave Irene for Judy, who (as expected) ultimately leaves him in return. Despite Irene leaving him time and time again, Dexter refuses to let go of his dream of ending up with her (at least until he learns from Devlin that she has lost her beauty and her passionate energy).
