Boxer represents the idealist, the true believer in the cause of Animalism. He is so good himself, so lacking in deceit or duplicity, that he can't imagine that others could backstab the worthy movement he is part of.
We appreciate Boxer as the moral center of this book. He is the kind of individual we call the "salt of the earth." He is hardworking, caring, dedicated, optimistic, and a good example to the other animals. The other animals look up to him and follow his lead, because they have seen his character firsthand and admire it greatly.
That is where the trouble starts. A true believer, goodhearted and pure himself, can be blind to the evil in his leaders. By throwing his weight behind evil without allowing himself to see the reality of it, Boxer leads others into being deceived by unscrupulous leaders like Napoleon. Orwell meant Boxer to represent the true, pure, goodhearted believer in the communist cause who was blinded by ideology and deceived into following a murderous tyrant like Stalin.