Lee uses words carefully to develop an arc for Boo Radley, but the narrative (the story) she tells is structured in three distinct parts. In this way, Boo is never entirely absent.
Boo Radley appears mostly in the first and last sections of the book, before and after the trial. He book-ends the trial as an example of how Scout grows and matures. When the Finch children are young, Boo is described in fanciful language.
Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained… (ch 1)
This is a children’s nightmare, just as the game to make Boo come out is childish. Yet Boo goes from being the unknown monster to more of a puzzle, as the children get gifts from him—including sewn pants and a blanket on a cold night.
During the middle section, the trial takes precedent. We are more concerned with Tom Robinson than Boo Radley. The two men are intricately connected though, structurally and thematically. Both are large physically, and strong and weak simultaneously.
In the third section, Scout has come full circle. This time, Boo Radley saves her life. We realize how carefully Lee has woven this plot, introducing us to the mysterious figure in the beginning and reminding us carefully here and there, only to have him play such an important role in the grand finale.
When Scout finally meets Boo, he has just saved her life and carried Jem home to safety. She is respectful and protective. She walks him home, and realizes what a role she has played in his life.
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them. Just standing on the Radley porch was enough. (ch 31)
The language describing Boo dramatically changes. In fact, as Scout looks out from the Radley porch, she summarizes the events of the book from Boo’s point of view. Just as Boo has always been a part of her life in the background, she has been secretly a growing part of his. They are now friends. Scout has grown up.