I think that Slim serves as a priest- like character for George. The manner in which Steinbeck describes Slim is laudatory, something that allows the reader to see Slim the same way that George sees him. Consider these excerpts from Chapter 2, when Slim first enters the narrative:
a majesty only achieved by royalty and master craftsmen...killing a fly on the wheeler’s butt with a bull whip without touching the mule... [According to Candy] 'Slim don’t need to wear no high-heeled boots on a grain team'... gravity in his manner and a quiet so profound that all talk stopped when he spoke....His ear heard more than was said to him, and his slow speech had overtones not of thought, but of understanding beyond thought...prince of the ranch... whose authority was so great that his word was taken on any subject.
If this is how the reader sees him, then, by definition, it is also how George sees him. George recognizes that Slim could help out both he and Lennie. In a setting where it has only been the two of them for so long, George sees a hopeful alliance in Slim. It is here where the friendship forms between both men. George, who has always done the thinking for both he and Lennie, finally recognizes someone who can provide much needed guidance in a setting where friends are few and adversarial threats are abundant. In the third chapter, we see this need for guidance develop more when George openly "confesses" to Slim what happened in Weed and the exact nature of the relationship between both he and Lennie. Steinbeck uses the idea of a "confessional" in quite a deliberate manner in this chapter because it helps to better understand the relationship between Slim and George. It only makes sense that when George needs some level of comfort at the end of the novel it comes from Slim reminding him that he "had to do it." In the end, it is the friendship between George and Slim that provides a fleeting moment of relief or guidance in an emotional world where nothing seems certain.
Slim is a mentor to George. It is shown through Slim's support of George and in his insightful, intuitive understanding of Lennie:"He's jes like a kid, ain't he" (p. 43). After George is forced to kill Lennie, Slim is the only one who understands what really happened, comforts George by assuring him he did the right thing, and suggests they go get a beer. Slim supports George when their jobs are endangered when Lennie crushes Curley's hand, even though he is shocked by the extent of Curley's injury. Slim actually endangers his own position at the ranch by threatening Curley, which emphasizes and accentuates the support he gives without asking for anything in return
George desperately needs some company. Sure Leny is always there but really how many dead mice can you throw away before things get boring? George loves Lenny but he is thrust into the role of care-taker for a big man with the maturity of a six year old. When they finally make it to the ranch George finally meets someone who he can relate to. Slim, like George, is young, hardworking and intelligent. Although Slim is Crew Chief, they both form a rare friendship. One of the larger themes of Steinbeck’s novella is the isolation and loneliness among these itinerant ranch workers. George and Slim share a rare friendship amidst the loneliness of ranch life. Slim even understands Lenny beyond the stereotypes and euphemisms given to him, "He ain't mean," said Slim. "I can see Lennie ain't a bit mean." Slim gently leads George and Lenny through the harsh realities of this particular ranch. This includes dealing with the boss’s son Curly, a man with a huge Napoleon complex. In the end Slim is there for George, even in the face of eventual disaster.