How can I create a good introduction to my essay answering "Which characters in To Kill a Mockingbird could the mockingbird be seen to represent?"
First, we should address what makes a good introduction. A good introduction has a thesis and what I call a road map. A thesis is the main point of your paper, and the road map is a small preview of how you will defend you thesis.
So, if your thesis is on the imagery of the mockingbird, then a good thesis might be:
While there are several noble figures in the book, there are two characters that embody the the definition of a mockingbird better than anyone else - Tom Robinson and Boo Radley.
After a thesis statement you will need to have a road map where you defend your thesis. It can be something like this:
Even though Tom and Boo only do good things for people, they are harmed by society.
Based on this road map, your first paragraph can be on what good things they do and your second paragraph can be on how they are harmed. By having these two elements, you are streamlining your essay and guiding your reader.