Scout is most definitely the prime protagonist in To Kill a Mockingbird. She is the narrator and central character. Her descriptions of life in Maycomb include many stories in which she meets with questions that need answers and people who clash with the world in which she lives. Many of the characters antagonize Scout: She has trouble with Miss Caroline at school; she fights with her classmates and gets an early learning experience about the low-life Ewells from young Burris; she is scolded by neighbors like Mr. Avery and Mrs. Dubose; she doesn't get along with her relatives (Aunt Alexandra and cousin Francis); and she becomes a target of Bob Ewell after Atticus discredits him at the trial. In the end, Scout learns from each and every one of her experiences, absorbing the new knowledge for future encounters.
The reason Scout is the protagonist in this story is evidenced by the changes that take place in her life over the two year period the book covers. She begins as a rather naive little girl who is not familiar with the lifestyles of those outside her household. Her world is the safe and comfortable confines of her home and father. She believes her father is a little boring and has few outstanding traits. She will find that he is the crack shot in Maycomb County and is not afraid of upholding truth and justice even when his peers do not agree with him. She finds other lifestyles, like those of the Ewells, the Radleys, Miss Maudie's, are not how she experienced life sheltered by Calpurnia and her family. As Scout learns, she accepts changes in her life that are beyond control. She begins to learn a female's role in society and how justice is not always just to those of color. Her acceptance of Boo Radley as a protector, not a gruesome monster, shows her understanding of people maturing.
Jean Louise Finch And Jeremy Atticus Finch are both protagonists and all of the other characters are antaganists.