Which character in the story does the author use to give an opinion that might direct us toward the theme?

If you’re talking about any story, then it would usually be the protagonist (the hero, the main character, and/or the humanitarian). On rare occasions the story will take place with a character that’s the antagonist (the bad guy, and usually has a lot of trauma that caused them to be the bad guy.) when the antagonist is the main character in a story it’s usually to show us the backstory of this person and what led up to them being bad.