Smitty is a character who doesn't actually appear in the novel and he is only referred to once, in chapter 2. According to Candy, he was a former ranch worker, a skinner like Slim, who was involved in an incident the previous Christmas when he started a fight with the stablehand Crooks. Crooks, being black, does not normally mix with the white workers but he was allowed in with the others that night, as it was Christmas. Candy tells George and Lennie that
Smitty took after the nigger. Done pretty good, too. The guys wouldn’t let him use his feet, so the nigger got him. If he coulda used his feet, Smitty says he woulda killed the nigger. The guys said on account of the nigger’s got a crooked back, Smithy can’t use his feet. (Chapter 2)
This is actually the first we hear of Crooks and his plight as an isolated disabled black man. His treatment here comes across as quite disturbing; it seems he is used to provide sport for the other men on a festive occasion.
Candy actually thinks well of Crooks; he has just called him a ‘nice fella’ (Chapter 2). However, in his relation of this incident, he does seem to share in the others’ casual racism, and is said to ‘relish’ the recollection of the fight.
As for Smitty, although we learn of him only briefly, we are given enough information to see that he compares with Curley in his small stature and pugilistic tendencies. His baiting of Crooks foreshadows Curley’s goading of Lennie.