Animal Farm is a socio-political allegory. As such, the author, George Orwell, is not as concerned with characterization as he is with making a statement about the development of socialism/communism in post-revolutionary Russia. So his characterizations of Napoleon, Snowball, and Squealer are rather simplistic.
- Snowball is characterized in positive way. We see him fight heroically at the Battle of Cowshed. He works hard to improve the lives of the animals. The idea of building the windmill leads to his eventual banishment, but this is only because it is such a good idea.
- Napoleon is characterized in a negative way. His only real goal is to consolidate power for himself. He kidnaps the puppies and turns them into a personal police force. He runs off Snowball and claims that the windmill was originally his own idea. He will end up violating all of the Seven Commandments of Animalism in some way.
- Squealer is also characterized in a negative way. He is simply Napoleon’s mouthpiece. He has no other function than to make the ruling pigs’ misdeeds seem acceptable to the other animals. He lies without remorse to accomplish this end. It’s true that he’s an accomplished and persuasive talker, but he never uses his talent for the welfare of anyone except the pigs.
Generally speaking, most great novels have characters with more depth, and we can expect to see both good and bad sides of them. But Orwell's goal with Animal Farm is to demonstrate something about a political system gone bad, so he doesn't want to focus on character development.