What is the connection of the symbol of the mockingbird and the characters in the story (including Jem, Tom Robinson, Mr. Raymond, and Boo Radley)?

In Chapter 10 of To Kill a Mockingbird, Atticus tells his children, "Remember it’s a sin to kill a mockingbird.” Scout, unused to hearing her father lecture her about very much, asks Miss Maudie to clarify what Atticus has said. Miss Maudie explains, “Mockingbirds don’t do one thing except make music for us to enjoy...That’s why it’s a sin to kill a mockingbird.” In other words, mockingbirds are defenseless creatures, which is why it's morally wrong to kill them.

Characters in the novel are as innocent as mockingbirds. For example, Boo Radley is roundly feared because he is developmentally disabled and shy, but he is in essence harmless. In fact, he is a kind man who rescues Jem and Scout in the end of the novel. Similarly, Jem is harmless, as he is a child, and so it is indefensible for Mr. Ewell to try to harm him. Tom Robinson is an African-American man in a racist southern town in the 1930s, and the trial against him and his death are unfair and unjustifiable. Finally, Dolphus Raymond lives with African-American people, so white people don't trust him. He, like the other characters, is as innocent and harmless as a mockingbird.