The most important character is Scout. Why? The whole story is told through her eyes. When you create a character, that character is developed by what they look like, what they say, how they act, and what others say about them. Throughout this whole book, we only see things through the eyes of Scout, who is a rather opinionated young lady. She tells us what to think about the other characters.
"Jem and I had considerable faith in Miss Maudie. She had never told on us, had never plaued cat and mouse with us, she was not at all interested in our private lives. She was our friend." (pg 45)
Miss Maudie was a favorable character throughout the whole book.
"Had I ever harbored the mystical notions about the moutains that seem to obsess lawyers and judges, Aunt Alexandra would have been analogous to Mount Everest: throughout my early life, she was cold and there." (pg 77)
Aunt Alexandra remains cold and aloof throughout the book.
"Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected on his abilities and manliness." (pg 89)
They respect and love Atticus, but it bothers them that he is that old. They think he can't do anything. They are surprised when they learn he can shoot a gun when he kills the mad dog. They question his effectiveness because he is so old,but they learn to respect his strength and knowledge.
Every character in the book is seen through Scout's eyes. We, as readers, evaluate and judge them based on what Scout tells us they do and say. She changes her mind on a few of them, but so do we when Atticus straightens us out (e.g. Mrs. Dubose).