The narrator is Jean Louise Finch, otherwise known as Scout in the story. She is Atticus's daughter. Scout is six years old when the novel begins and is eight when it ends. However, the narrator is a much older, mature Scout. She is an adult telling the story as she remembers it as a child. Since the narrator is an adult, the reader has a better understanding of the lessons and events that Scout went through as a child. This is a technique that Harper Lee uses to get an adult's viewpoint through a child's voice.
The narrator of the story is Scout. Her full name is Jean Louise Finch. The story takes place from 1933-1935 when Scout was six to eight years old. What is most important to note is that she is telling the story as an adult. So, even though she is writing from a first person account, she is recounting the events as an adult.
Scout is not an omniscient narrator, but she is reliable, because she is telling the story from retrospect. The time difference, unfortunately, is never given in the novel.
Another important point to keep in mind is that Scout is recounting first-hand events. Not only did the events of the book take place in her life, she was at the center of it. For example, Atticus, the one who defended Tom Robinson, was her father. From this perspective, what we have is a reliable narrator who lived through the events and who is speaking from the vantage point of maturity.