Curley's wife, as the only woman on the ranch, occupies a key role in the narrative as the catalyst for much discontent for several of the characters and, of course, for the tragic ending.
The fact that she remains nameless throughout the novel is important: her identity is significant only in the context of her marriage to Curley, and she brings nothing of value to the marriage in part because she doesn't value herself. Although she is a good-looking woman, she is intensely lonely throughout the novel--even though surrounded by men--and she epitomizes an unrealized life. She exists to tempt, to insult, to hate everyone around her, including her husband, and she is so self-obsessed that she can't even see how others perceive her.
When she first sees George and Lennie, she pretends to be looking for Curley, and when George tells her that Curley isn't there, "she smiled archly and twitched her body. 'Nobody can't blame a person for lookin," she said." Her body movements, and her comment about looking, appropriately sum up her role in the narrative: she is constantly looking for something different, for something better, that's going to bring some meaning to her meaningless life.
She's also bitter and cruel, trying to bring down the men to her own level when she sums up the group as "a nigger an' a dum-dum and a lousy ol sheep." Candy's response to the wife's tirade is almost gentle: "Maybe you just better go along an' roll your hoop. We ain't got nothing to say to you at all." For Curley's wife, this is a devastating response because it tells her that, despite her looks and enticing body language, the men send her away as if she were a small child playing with a rolling hoop.
About the only time that she becomes a three-dimensional character, ironically, is at the end when she's trying to explain herself to Lennie. She's as self-absorbed as ever, of course, but we do feel her loneliness and sense of wasted life. Her self-absorption, however, leads inevitably to her death because she simply doesn't take the time to understand the implications of her actions--a lack of awareness that pervades her character throughout the novel.