The boss is largely an invisible figure. He emerges in the beginning of the story and after that he disappears.
When George and Lennie arrive late, Candy is quick to point out that the boss is not happy. In fact, Candy also says that the boss will give the stable boy hell. No reason is given for this action of the boss, other than the fact that the stable boy is black. It seems that the boss is a racist man who takes out his frustration on black people, because of the color of their skin. Here is the quote:
"I guess the boss’ll be out here in a minute. He was sure burned when you wasn’t here this morning. Come right in when we was eatin’ breakfast and says, ‘Where the hell’s them new men?’ An’ he give the stable buck hell, too."
Is the boss nice? From the above point, no. He is a racist man. But most men in this context are probably racist.
Candy says that the boss is nice owing to the fact that he tries to be nice to the men at times. For instance, during Christmas, the boss gave the men a gallon of whiskey. George is impressed with this point as well.