In chapter 2, Curley's wife first appears. She comes into the doorway and claims she's looking for Curley. Now, she'd come from the house and that's where Curley was headed. This is somewhat suspicious since they would likely have crossed paths. But, Curley's wife is quite lonely, sitting in the house all day, so she might just be looking for company. She is also clearly flirtatious. Slim, having been at the ranch for some time, knows how to deal with her and with Curley.
Lennie immediately thinks she's pretty and George recognizes this as trouble.
Well, you keep away from her, 'cause she's a rattrap if I ever seen one. You let Curley take the rap. He let himself in for it.
Crooks and Candy also see Curley's wife's presence as asking for trouble. Not only will talking to her incur the jealous wrath of Curley, but since she is the boss's daughter-in-law, their jobs are also at stake.
In Chapter 4, Candy tries to protect Lennie (this is after Lennie has crushed Curley's hand). Crooks scolds Curley's wife and asks her to leave. She fires right back, threatening to hang him. Crooks knows this could happen, but Candy defends him as well. As she is leaving, she makes a last effort to connect with Lennie as she clearly sees he is the one she can manipulate.
I'm glad you bust up Curley a little bit. He got it comin' to him. Sometimes I'd like to bust him myself.
Although Curley's wife seems like nothing but trouble here, she is similar to Crooks, Lennie, and, to a certain degree, Candy. Curley's wife is a woman alone on the ranch, while her husband is out at the whorehouse. One can't help but be sympathetic despite her treatment of Crooks in this chapter. Crooks is outcast because of his race. Candy is old and Lennie is always out of place because of his intellect. Interestingly, in chapter 4, the four outcasts find themselves together while the other men are at the whorehouse.
In the end, Lennie is the one Curley's wife gets relatively close to (aside from Curley) and this proves to be disastrous when Lennie panics.