Myrtle Wilson, Tom Buchanan's woman on the side, who is alluded to by Jordan Baker in the first chapter, comes fully on stage in Chapter II, when Nick and Tom visit Wilson's garage on the edge of the Valley of Ashes. From Nick's first viewing of her, to the scene in the New York apartment, she changes clothes three different times, and Nick refers to this as a "costume" (35) change. This always makes me think of the expression "Clothes make the man," or in this case, the woman. But the word "costume" is central to the analysis, showing us that while Myrtle may don a costume that changes her personality, she is still from the Valley of Ashes and not part of Nick and Tom's world of wealth.
When Nick first meets Myrtle, she is in a "dark blue crêpe-de-chine" (29-30) dress, which is a silky kind of material. Nick finds nothing about her to be beautiful, but he describes her having "an immediately perceptible vitality abut her as if the nerves of her body were continually smoldering" (30). She is a sensual and vital woman in this scene.
Myrtle changes clothes for the train ride to Manhattan, to a "brown figured muslin which stretched tight over her wide hips" (31). This is not a very attractive picture, but perhaps this was a sensible choice for the train ride, or it is what she considers suitable for the train ride. We are also left with the impression that Myrtle enjoys changing "costume."
At some point after Tom, Nick, and Myrtle arrive at the apartment, Myrtle has made her costume change, "attired in an elaborate afternoon dress of cream colored chiffon which gave out a continual rustle as she swept about the room." (35). Nick notes that Myrtle has changed not only her dress but also her personality. He says, "The intense vitality that had been so remarkable in the garage was converted into impressive hauteur" (35). Her gestures and movement become more expansive and even her voice changes "to a high mincing shout" (35).
Myrtle is trying to play the role of lady of the manor, to impress her guests, her sister Catherine and a neighbor from the apartment below, as well as Nick and Tom. She believes that the clothes will make the woman, and she also believes that her clothes make her equal to Daisy. But by the end of this scene, it is clear that she is still Myrtle from the Valley of Ashes, not even permitted to utter Daisy's name in front of Tom, who hits her and breaks her nose when she presumes to do this.