How does Steinbeck Portray the Character of Slim in Of Mice and Men?
Here’s a collection of quotations with analysis for the character of Slim.

Slim is portryed as an almost priest-like authority figure: ‘the prince of the ranch‘, with hands that moved like a ‘temple dancer’. Of all characters, he seems most clear sighted, most correct: most like the author’s point of view, in other words. His dramatic function is:

  1. for George to make his ‘confession’ to about why he travels with Lennie;
  2. as the personification of natural, honest authority, NOT the hard authority of the Boss, or Curley who abuses his position as ‘the Boss’ son’. Slim’s ‘authority’ ‘was so great that his word was taken on any subject.’ 
  3. Therefore, if Slim says Lennie is a ‘nice guy’ then we see Lennie as a ‘nice guy’ (though he’s a murderer).

In more detail…
Slim represents justice in a vigilante society. He defends George and Lennie against Curley: “you just try and get this guy canned”.
As the voice of Steinbeck… He persuades Candy that it is right to put the old sheepdog out of its misery. If Slim says it’s right, then it must be. This is a bit unsettling as it looks like Steinbeck is making a case for euthanasia (as Slim is normally closest to author point of view).
Slim also seems to give similar hints to George. ‘Suppose they lock him up… that ain’t no good”. He stalls for time so George can do this, saying to Curley: “maybe you better stay here with your wife”  after she’s been killed. 
Slim as Masculine Ideal…This romanticised view of a hero, explored by Steinbeck, perhaps originates in the ‘Wild West magazines that ranchmen love to read and scoff at and secretly believe.’ He wears a ‘Stetson’ and ‘denim jeans’, fashion typical of the time, and when we first meet him he is combing his ‘long, black, damp [just washed] hair straight back’ showing he is clean and well groomed. He’s also handsome, with a chiselled jaw and his ‘hatchet face was ageless’, this timelessness coupled with his ‘majesty’ give him an air of mystery and heroism that was being promoted in movies at the time as Hollywood began to establish itself. He also contributes to Steinbeck’s symbolic use of hands to portray character in the novel as his are revealed to be ‘large and lean’ but ‘as delicate in their action as those of a temple dancer’.
Without Slim in the novel, our picture of working men in this  novel would be rather grim. The fact that Slim is altruistic for no other reason than that he is a genuinely ‘good guy” is reassuring and suggests that despite all the hardships and exploitation that travelling workers faced, trust and brotherhood did exist. Slim is important as he shows that Curley’s wife is not entirely right when she says that “men are all scared of each other. Ever’ one of you’s scared the rest is gonna get something on you.” Slim is loyal and trustworthy and is not afraid of anyone in the novel.