I think Atticus Finch represents justice. On a surface level he is a defense attorney, so it is his job to provide the best possible defense to his clients so that justice might be faithfully carried out. On a deeper level though, Atticus's character is a model of justice. The book takes place during a racially unjust period of history. Black and white cultures are not fully integrated and racism runs deep for many people still. Even in the town of Maycomb there are plenty of people who believe Tom is guilty simply because he is black. Atticus, on the other hand, doesn't believe in judging a person based on his or her skin color. He treats all people fairly, with respect, dignity, and the honor they deserve for their actions, not their skin color. Atticus tries to instill those beliefs and behaviors in his two children, Jem and Scout.
I also think Judge John Taylor deserves a nod for justice as well. Again, there is the surface interpretation. He's a judge of the courts, so he deals out justice every day. But the Tom Robinson case shows that Taylor really believes that justice needs to be given and defended at all costs no matter the skin color. Taylor specifically appoints Atticus to the case, even though a standard court appointed attorney should have been given the case. Taylor gives Atticus the case, because he believes that Atticus is Tom's best chance of a not guilty verdict. Taylor knows Tom didn't do it, and he wants to see justice served fairly. And he does it by working within the system that he upholds and works for.