The boss is a minor figure in the book.  Therefore, there is not much information on him. 

First, he is angry.  He is angry, because Lennie and George were supposed to come the day before.  So, when they were late, he was awfully angry.  Candy relates this fact to Lennie and George.

“The boss was expectin’ you last night,” the old man said. “He was sore as hell when you wasn’t here to go out this morning.”

From these words, we can say that the boss probably has an anger problem.  This is confirmed, when he takes out his frustration out on Crooks, a black worker.  This also shows that boss suffers from racism.

Second, the boss is not completely one dimensional.  There is a nice side. Candy says that at Christmas the boss gave the men a whole gallon of whiskey. When George heard this, he was surprised.

“Well, he’s a pretty nice fella. Gets pretty mad sometimes, but he’s pretty nice. Tell ya what—know what he done Christmas? Brang a gallon of whisky right in here and says, ‘Drink hearty, boys.

Finally, the boss is a suspicious man.  He questions George and his relationship with Lennie.  In particular, he questions why George sticks around Lennie. Is he taking Lennie's money?  In any case, the boss says that he will keep an eye on them.