Curley's wife is characterized in several ways in Of Mice and Men.
She is discussed by other characters when she is not present, which serves to establish a "hear-say" about her. This discussion leads to a definition of her character as "a tart" and a trouble-maker.
The unstated reason she is seen as a trouble-maker is that the men find her attractive and so are concerned that, despite their better judgment, they may be taken in by her.
Curley's wife is also defined by her actions. She repeatedly comes into the bunk house and other places where the men might be, while saying that she is looking for Curley. This is a lie and it is presented as a lie in the novel. The simply duplicity of the obvious lie helps to define Curley's wife as dishonest and unintelligent.
Finally, she is defined by her own speech. At this point, when speaking with Lennie, Curley's wife becomes a sympathetic and even pathetic character who is seen to have little to no control over the circumstances of her own life.
Additionally, Curley's wife is never given a name. This lack of information also serves to characterize her as holding a marginal status on the ranch.