Curley is the son of the owner of the ranch where George and Lennie go to work. He is extremely hostile throughout the whole time that we see him.
The most obvious example of this is how he treats Lennie. He starts to dislkie Lennie and to bully him the moment he sees him. First, he tries to insist that Lennie speak for himself when he clearly doesn't want to. More importantly, he comes after Lennie and starts beating him for no reason. He only stops when George finally tells Lennie to grab Curley's hand.
Curley is also very hostile everytime he shows up looking for his wife. He is never polite and is usually pretty confrontational.
Curley is a bully who looks for trouble and weaker people to humiliate. He is the bosses' son and he uses his position to push others around. He has a friend who is as bad a bully as he is named Carlson.
Carlson starts up on Candy's dog. Noises are heard outside. Slim goes to call the boys in for supper. Lennie is excited about the prospect of a puppy and is asking about having a brown and white one. As Lenny gets off the bun, Curley bounces in the door. He is looking for his wife and already angry. When George tells him that she was looking for him, he turns on George to size him up.
"His eyes flashed over George."(37)
"Curley scowled at him, and turning, hurried out the door."(37)
Later Curley is again looking for his wife at the bunkhouse.
"Curley looked threateningly about the room."(53)
Curley later targets Lennie and gets his hand broken.