Slim is a skilled mule driver who is referred to as the prince of the ranch. He is a mule driver for the ranch and everybody goes to him for advice. Slim is a quiet man who understands a lot of stuff. Slim understands how George and Lennie were with each other and he comforted George at the end of the story.
The most distinguishing aspect of his personality is his selflessness and sympathy, as well as his sense of honour. He is sympathetic towards George for having to look after Lennie and is the only person that Curley directs any true respect. Ironically, Slim is absolutely everything that Curley isn't: Charismatic, selfless, peaceful, kind, loyal, highly intelligent, calm and fearless. He was good-natured and very tolerant, his cool demeanour remaining intact almost throughout the story. Slim was kind to animals and drowned four of his pups when his pet dog couldn't feed and sustain all nine. He is also kind and respectful to all of the disliked characters in the book: He treats Crooks with total sympathy and gives Curley's wife the respect she deserves and so sorely wants. The one time he loses his cool is when Curley confronts him, suspicious that Slim is fraternising with his wife. He rather sensibly and perceptively comments that Curley is completely unable to look after his own wife.
Slim is shown respect at all times throughout the novel and we see him protecting Lennie from Curley in the film when the fighting happens.