Slim is one of Steinbeck's functional characters in Of Mice and Men. He quietly encourages others to confide in him because of his discretion and tolerance, qualities lacking in all of the novel's other characters.
Slim provides absolution to most of the characters by not judging them. He rarely says anything negative about others, and his actions provide security for those who might need absolution. A couple of examples include:
1. When George discusses Lennie with Slim, he feels confident enough in Slim's character to tell him about the incident in Weed and about cruel jokes that he and others used to play on Lennie. Slim does not judge Lennie for the incident in Weed. He says that he can "tell a mean guy a mile away" and that he knows Lennie isn't mean. He also doesn't seem to hold George's pranks against him.
2. When Lennie breaks Curley's hand in Chapter 3, Slim forces Curley to promise not to tell anyone what really happened to his hand. He protects Lennie from Curley's revenge and provides George with the comfort that he and Lennie can stay at the ranch.
3. Finally, at the end of the novel, Slim is one of the only men (with the exception of Candy perhaps) who knows why George shot Lennie. Aware of the ramifications of this act upon George, Slim tries to comfort him by telling him that there was nothing else to be done and offering to go into town with him for a drink. He demonstrates that he not only understands George's decision but also that that choice would not discourage him from being a "drinking buddy" with George.