Lee does not introduce Boo directly. She first introduces his actions, saving the children, and then Scout notices him. By keeping him in the background and then bringing him out, Lee mirrors the structure of Boo’s story.
Boo begins behind the scenes. In the beginning of the book, Boo Radley is more of a myth than a person. As the story continues, Boo begins to materialize as less of a monster and more of a kind person. He leaves presents for the kids, and puts a blanket on Scout’s shoulders when she is out at the fire.
Boo begins trying to protect the kids early on. Perhaps he realizes that they are interested in him because they want to be nice to him. They feel sorry for him for being cooped up.
When Jem leaves his pants behind, Boo Radley sews them up to try to keep Jem out of trouble. When Bob Ewell attacks the children, Boo Radley is there to come to their defense.
The first mention of Boo is a description of him. Scout points to him, and then sees him.
He was still leaning against the wall. He had been leaning against the wall when I came into the room, his arms folded across his chest. (ch 29)
When he is described, he is all pale and white. Much imagery is used.
A strange small spasm shook him, as if he heard fingernails scrape slate, but as I gazed at him in wonder the tension slowly drained from his face. His lips parted into a timid smile, and our neighbor's image blurred with my sudden tears. (ch 29)
Scout mentions that his appearance is like her fantasy coming to life.
His ghost-like descriptions match his ghost-like appearances in the story. Yet he is also kind and gentle, and when Scout brings him home she looks at things from his point of view, from his porch.
Boo Radley comes is introduced in the last chapters as he is introduced in the book, left in the background and then brought out briefly.