Which animals character didn't seem to change at all since the rebellion?

That would be Benjamin the donkey. The Rebellion does not change Benjamin’s personality, although he eventually helps the animals read the lettering on the side of the van and the maxim that replaces the Seven Commandments. Benjamin represents the human (and also stereotypically Russian) tendency towards apathy; he holds fast to the idea that life is inherently hard and that efforts for change are futile.