There are several characters that you will study as you're reading the book; George and Lennie, Candy, Crooks, Slim and Curley's wife.

However there are some other characters who are not main characters, but they're still important because they represent some of the the themes in the novel, and the tell you a lot about what life was like at the time and in the place that 'Of Mice and Men' is set.

Curley's a mean little man with a short temper whose father owns the ranch that the men work on.  He wears boots like his father's, but with a little built up heel toy make himself look a little taller.  He used to be a boxer, and was pretty good at it, and he's got an inferiority complex about being small so he likes to pick fights, particularly with bigger men.   Curley's wife says he's 'not a nice guy', and we see that he's a bully when he tries to pick on Lennie because he thinks Lennie's weak.  That's until Lennie breaks his hand of course.

Whit is important for one incident. He shows the other ranch-hands a letter in a magazine, written by a worker he had known on the ranch previously. He relishes the memory of this man (Bill Tenner) and shows his own loneliness, and longing for friendship; yet even as he shows the magazine to George, he will not let go of the page.  Bill's dream waas to have a letter published in the magazine, and this is the only occasion in the story that we hear of someone's dream being realised.

The Boss
The Boss appears briefly, voicing suspicion at George's speaking for Lennie.  This moment is important because it demonstrates how uncommon it is that two men would be travelling together and looking out for each other.  The Boss assumes that George is taking advantage of Lennie, stealing his money, showing people did not trust one another in these times.

Carlson is typical of the men George describes as “the loneliest guys in the world”. He is outwardly friendly, but essentially selfish. He finds the smell of an old dog offensive so the dog must be shot. He has little regard for the feelings of the dog's owner. At the end of the story, as Slim goes to buy George a drink, and comfort him, it is Carlson who says to Curley, “What the hell...is eatin' them two guys?” He simply doesn't understand the relationship that George and Lennie have in the story.

Girl in the Red Dress
The reader does not meet the girl in the red dress in the novel, but we hear about here in the story that George tells Slim.  From that story we learn that George and Lennie had to leave their last place of work because Lennie tried to touch the girl's soft red dress and the girl screamed, thinking that Lennie was attacking her.  The reader knows Lennie by this pointg in the story and we like him, so to the reader the girl seems dangerous and perhaps a little silly to have misread Lennie's intentions.  It isn't a coincidence that Steinbeck put the girl in a red dress, and she certainly represents danger for George and Lennie who would have been shot had they not left town.