Of Mice and Men

Slim represents the American dream of a meritocratic society. Although Curley has some authority as the boss's son, slim has everyone's actual respect as the 'prince of the ranch' and moves with a grace only found in 'royalty and master craftsmen'. He is a skilled jerkline skinner (drives mules by using long reins and they drag carts or a plough or whatever). He's a skilled man and a good person, but even he can't escape the system of dead-end itinerant labour and lives in the bunkhouse with the rest of the guys despite being a mature man who should be able to have his own house and family if the American dream actually worked.
He's also a plot device - he's a listening ear who allows George to explain his feelings so the reader can hear George's own thoughts in his own words.
He represents justice as well - he runs the bunkhouse, is fair and pleasant to Curley's wife and Crooks, deals with the situation when Lenny crushes Curley's hand, and ultimately deals with Lenny. The fact Slim - the most upstanding and kindly figure on the ranch, and a counterpoint to the *****y, cruel characters in the story - is the one to shoot Lenny makes the ending even more poignant and depressing.