To me, it seems you are trying to write an essay that may have 3 body paragraphs, one about Atticus, one about Bob, and you're considering Miss Maudie for the 3rd, but are open to other characters.
If you are looking for someone exactly in the middle of good and evil, you should use Boo who by all evidence appears evil, but by the end, Scout sees and learns that he is indeed good. And this is a learning experience for her as she experiences him saving her brother, and stands on the porch looking through his eyes or perspective. This way you could take Atticus' notions about walking in someone's shoes, or walking around in someone's skin and work that throughout your paragraphs from two different vantage points: Atticus' instruction, and Scout's experience of this moral value (through Boo).
Another idea I have for you is to take Maudie's ability to education those children morally as an entire paragraph. She used the cakes to entice them to dialogue with her about the value of what their father said and who he was. She used simple gesture at the Missionary Tea to help Scout know not to say anything bad. Through example, she endured the loss of her house. She held her head up and kept going. She didn't let it get her down. She says really neat things about Atticus that allude to the fact that Atticus does the right thing while the rest of us pretend to. She slams the religion of some as ritual wherein it should be experience. This is a tremendous lesson to the kids because anything worth doing is worth doing right.
I hope reading between the lines of your request actually hit a little of what you were looking for.