The entire novel is narrated by an older Scout Finch. She is an adult remembering her childhood in Macomb, Alabama.
Because of this, the entire story follows her perspective only. As readers we only know about the plot and other characters through her eyes. She is also very young in this memory, only five at the beginning of the book, which some have argued could make her an unreliable narrator given the fact that children perceive the world very differently because they are so young and naive.
Whether we view Scout as reliable or not as a narrator, the fact remains that this is HER memory. The events she includes, the details she provides, the emotions she remembers feeling are all from her unique perspective. If this book had been narrated by multiple characters, it would have been a far different story simply because the perspectives would have changed all of those elements.