Is Slim a lonely character? How?
It seems like Slim and George have each other to talk to. They get along pretty well. Does that mean that, after Lennie died, George will have Slim (who is on the same mental wavelength as him) to talk to? So they won't be lonely?
I think, in an ideal world, that is what would happen. Slim is the only person who truly understands what George did and why he had to do it. He is the only person in whom George can confide. Slim seems to have seen in George and Lennie's relationship something to envy and something that is missing in his own life.
However, given the nature of the itinerant farm worker characterized in this novel, it's more likely that George will finally become more like the other men around him. He will probably continue to do his job but his goal for the future will be greatly different. He will work to earn money to entertain himself and not much more.
I don't think Steinbeck holds out much hope for the lonely in this novel. Sadly, I think that George and Slim will never develop a deep friendship that will prevent them from being lonely.
Slim is a lonely character in the sense that not many in his world match his level of skill, intelligence, and sensitivity. He is described in narration as godlike in comportment; when he speaks, men listen. Even Lennie, whose memory is terrible, remembers Slim. When George questions Lennie about Slim and Curley's wife possibly being in the barn, Lennie is able to tell George that Slim had been alone, and had told Lennie not to pet the pups so much.
Perhaps the only character in the novel for whom Slim felt companionship was, indeed, George, but as the answer above states, George seems primed to go about the rest of his life in the way he described to Lennie in part one: in cathouses, drinking whiskey and shooting pool.