Slim, the jerkline skinner, is tall and strong, yet gentle in his movements and speech, and he is very good at his job. However, even his impressive physical qualities are not his most important attributes.
His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought. (chapter 2)
In this introductory description Slim is presented as a man of superior understanding and wisdom. He looks deeply into any situation or problem; he is able to see beyond the immediate and the obvious, and he is an excellent judge of character.
The other men, whether consciously or unconsciously, all look up to Slim. They turn to him for advice and support. George, although just newly arrived, confides everything to him, and even the belligerent Curley never dares to go against his wishes.
Most importantly, it is Slim that helps to decide the conclusion of the story. He counsels George to deal with the fugitive Lennie himself rather than allowing anyone else to hunt him down, or institutionalise him. Slim realises that it is best if George kills Lennie, as he will do it out of compassion, and Lennie will die happy this way.