Slim is described always in terms of dignity and majesty. When he first comes into the bunkhouse, he moves "with a majesty achieved only by royalty and master craftsmen. He was a jerk-line skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders." Slim is tall, ageless, and an expert in his job. His voice is the voice of rationalism. When Carlson suggests killing Candy's dog, Candy appeals to Slim as the final authority.
Slim is so respected and admired on the ranch that even Curley listens to him. When Lennie smashes Curley's hand, Slim is the one who intercedes and tells Curley he will not have George and Lennie fired. Slim understands Curley's fear of ridicule, and he uses that fear to help George and Lennie. Slim also inspires confidences because he is not judgmental. When George first meets Slim, George tells him about Lennie's troubles in Weed. George senses in Slim a person of intelligence and empathy who will not be mean to Lennie, make fun of him, or take advantage of him.
Slim is the only one on the ranch who appreciates the difficulty of George's position. He understands the constant oversight George must exercise in watching Lennie and keeping him out of trouble. It is Slim, in the end, who suggests that George did the right thing in killing Lennie mercifully. He explains the alternative: "An s'pose they lock him up an' strap him down and put him in a cage. That ain't no good, George."
Slim is present at every crucial juncture in the story: the death of Candy's dog, the smashing of Curley's hand, finding the body of Curley's wife, at the pool after George has shot Lennie. In each case, there is violence or the threat of it. Each time Slim helps make the assessment to do what is merciful or what is right.