Which of these characters in George Orwell's Animal Farm is an anthropomorphic character?

A good definition of anthropomorphism is giving human qualities to other things, whether these be gods or animals. In the case of Animal Farm, we are dealing with animals.

So, we can say that the whole book is an anthropomorphism. It plays the most important role in the book. At first the anthropomorphisms are small. The animals speak, some well, others not so well. Then the anthropomorphism becomes more significant. Some animals are able to read and they become very eloquent like Squealer. Then the animals are able to build things like windmills. Finally, in the last few pages of the book, the pigs turn into humans. In this sense, the anthropomorphism is complete. In fact, there is a bit of irony. The anthropomorphism is so complete that is it no longer an anthropomorphism, because the pigs are now humans.