There are several reasons why Atticus is unbiased:
1. He tells Scout to see things from other people's perspectives. This is seen towards the beginning of the novel with Scout's teacher--Miss Caroline. She was not from their area, so she did not know the people in Maycomb.
2. Boo Radley--at the end of the novel Scout finally realizes what Atticus meant by seeing things from someone else's point of view. She sees how Boo saw the kids/events that happened to her, Jem , and Dill over the years. He only wanted to help them/befriend them.
3. Tom Robinson trial--Atticus was appointed to take this case. He believed in Tom's innocence, and he did all he could to help Tom. For example, he clearly showed through evidence in the trial that Mayella was beat by her father.
4. Mrs. Dubose--even though she had her own views about Atticus defending Tom Robinson (she was racist and believed that Atticus was trash for defending Tom), Atticus only treated her as a lady. He knew she had a morphine addiction. He thought she was the bravest person he knew--she had moral courage.