Who is slim in Of Mice and Men?

Slim is a very small but significant character in 'of mice and men'. He is the foreman on the ranch owned by Curley's father, without him we wouldn't see the difference between how the "ranch hands" respect Curley to how they respect Slim. He is known as the "top-dog" among the workers on the ranch-they all really respect him and see him as somewhat godlike, there's also something mysterious about him.