You can discover the narrator of Rodman Philbrick's novel Freak the Mighty in the very first sentence of the book.
I never had a brain until Freak came along... (Chapter 1: The Unvanquished Truth)
The novel is narrated in the first person, and one of the main characters--Maxwell Kane aka The Mighty--is telling the story from a retrospective point of view. The events of the novel have already occurred, and Max retells the story about him and Kevin and how the two boys united to become Freak the Mighty. Max, who hates to read or write and who suffers from a lack of confidence through much of the novel, decides to put his story down on paper,
So I wrote the unvanquished truth stuff down and then kept on going... (Chapter 25)