To answer this question, take a look at chapter two, when George and Lennie arrive at the ranch. What you'll notice about Candy's description of the boss is that it is both positive and negative.
For example, Candy mentions that the boss was very mad about George and Lennie not arriving at the ranch in time. He also says that the boss often gives Crooks "hell" whenever he is mad. This suggests that the boss is not a particularly nice person and that he is racist.
However, Candy also says that the boss is a "pretty nice fella." To support this idea, Candy tells George and Lennie that at Christmas, he gave the men a gallon of whisky as a gift and even let Crooks come into the bunkhouse.
Based on Candy's description, then, the boss is portrayed as being kind toward the men at Christmas but having a bad temper, which is frequently felt by Crooks—the only black man on the ranch.