In the novella, Slim is an interesting character. He is one of the only men to treat Lennie kindly and considers Lennie a good person. From the beginning, we learn that Slim is "the prince of the ranch" - ie: he is the one everyone looks up to. He shows himself to be a good listener, as seen when George confesses how he met Lennie and how he used to play cruel jokes on him. Slim listens but does not judge.
Steinbeck describes his eyes as "god-like", meaning he is almost omnipotent, seeing clearly everything that goes on and who everyone really is. He is honest and fair and one of the best workers on the ranch.
According to Steinbeck in Chapter 2, "…he moved with a majesty only achieved by royalty and master craftsman. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders. He was capable of killing a fly on the wheeler's butt with a bull whip without touching the mule. There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke, His authority was so great that his word was taken on any subject, be it politics or love. This was Slim, the jerkline skinner".
Slim is quite the character. He is seen here as an extremely capable mule driver (skinner) and skilled with a whip. But he is a quietly intense man who has the final say on what goes on at the ranch, or any topic really, because the other men trust him - as seen with Candy and the killing of his dog.
It is also important to note that Slim is the only character who truly understood why George had to kill Lennie.