In what ways do Tom, Boo, and Mayella portray a mockingbird?

Tom- simple man who tries to help people. He cannot understand all the forces that mean him harm.

Boo- Shy introvert that has had much trauma in his life. He loves children and lurks in the shadows looking out for them. Too much attention would destroy him.

Mayella- an emotionally and mentally fragile girl who is being abused by her father. There are clues that she has a kind heart.