Crooks, like Candy, Lennie, and Curley's wife, is a person who bears a proverbial mark. Along with these other figures, Crooks is kept far from any position of power. Because of his skin color, Crooks is discriminated against and isolated.
Crooks, the despairing old Negro stable worker, lives alone in the harness room, ostracized from the ranch hands.
His isolation, though stemming from a different source, is quite similar to that of Curley's wife. The kinship between the outcasts becomes clear in the story when Crooks, Lennie, Candy and Curley's wife are all together for a moment in the stable room where Crooks lives. Briefly, they share a hope to overcome the divisions that separate them, but this hope is quickly extinguished as they bicker back and forth.
The reality of isolation is too great to overcome.
The presence of Crooks in the story suggests yet another mode of isolation and another social division that defines the society of the time.
In their world, isolation is the norm.