In John Steinbeck's Of Mice and Men, what character(s) foil each other in comparison or contrast?
I do not believe that Steinbeck was interested in creating characters that were striking for the ways in which they were either similar or different. George Milton and Lennie Small are obviously very different, but the other characters are pretty much unique individuals. Steinbeck's main purpose in writing this short novel, which he intended to convert into a stage play (see Introduction in eNotes Study Guide), was to show the different kinds of men who worked on ranches in California. He wanted to create a spectrum, a variety. It would be very hard to pick any two characters--other than George and Lennie--who were intentionally created to contrast or compare with each other--although it would be easy enough to say that Crooks and Candy are alike in being crippled, or that Slim and Curley are different because Slim is rational and considerate while Curley is emotional and a bully.
Steinbeck knew he had to keep his cast of characters limited, not only because he was writing a short book but because he understood that too many actors on a stage would only confuse his future theater audience. His main characters are George, Lennie, Candy, Crooks, Carlson, Curley, and Curley's wife. They have to represent a spectrum of the kinds of people one might meet on a California ranch in the 1930s.
Geprge and Lennie are buddies but they contrast in many ways. As Slim points out to George:
"Hardly none of the guys ever travel together. I hardly never seen two guys travel together. You know how the hands are, they just come in and get their bunk and work a month, and then they quit and go out alone. Never seem to give a damn about nobody. It jus' seems kinda funny a cuckoo like him and a smart little guy like you travelin' together."
George is a little guy. Lennie is a big guy. They have a symbiotic relationship. Lennie can give George protection. He is like a big watchdog who will do anything George tells him to do. Lennie can be a great asset to a little guy who has to travel around in boxcars among hungry and potentially dangerous men and sleep in the open. At the same time, Lennie needs George to get him jobs and to tell him what to do and what not to do. The one big thing they have in common is their dream of owning their own little farm. George is the only one who has enough brains to acquire such a farm, and Lennie would be a tireless worker.
Note that in Slim's description of itinerant farm workers, he emphasizes that they "never seem to give a damn about nobody." It would be hard to compare or contrast such men. They are mostly loners. George and Lennie contrast with all the other men in being friends who share the same thoughts and feelings.
Carlson does not seem to stand out as a character. He is included mainly for the purpose of explaining how George gets hold of a loaded pistol when he decides to kill Lennie. The business about Carlson shooting Candy's dog was only created to demonstrate that Carlson possessed a German Luger (suggesting that he was a World War I veteran and had brought the weapon back from Europe). George watches Carlson cleaning the foreign automatic after shooting the dog, so George knows how to use it when the time comes. The gun has to have a distinctive shape so that the audience will realize without being told that George stole Carlson's Luger with the intention of shooting Lennie at the riverside campsite.