Boo Radley is a subtle, yet very important character to the structure of the story of "To Kill A Mockingbird". We can learn a lot about him by regarding him from the viewpoint of several other characters in the book.
His own family, the Radley family, are ashamed of him and disgraced by him. When Boo was involved in some 'antics' as a fifteen year old, Mr. Radley refused to send him to the state industrial school even though it wasn't considered a disgrace and was certainly better than prison. Mr. Radley instead kept him at home and didn't release him from his improvised prison. "....Mr. Radley's boy was not seen again for fifteen years." When the incident with the scissors came, and it was clear that Boo was disturbed in his mind, Mr. Radley wouldn't admit that he was crazy. Just highly strung, he said. He stated that 'no Radley is going to any asylum' and so he kept him locked up in the house.
Even through the time-span of the book, Nathan Radley (Boo's brother) tried to keep Boo away from other people and to cut off all association with anyone else. An example of this is when he cemented the knothole of the tree to prevent Boo from leaving presents for the children.
Maudie Atkinson is a pragmatic character. When Scout asked her if she thought Boo was still alive, her affirmative answer was due to the fact that she 'hadn't seen him carried out yet'. She remembers Boo when he was a boy and that he always spoke nicely to her. She thinks that if he wasn't crazy then he is now. However, she is not prejudiced against him because of it and still speaks of him with respect, e.g. calling him Mr. Arthur Radley instead of Boo. Miss Maudie has nothing but pity for Boo and calls the Radley house a 'sad house'.
Atticus also speaks about Boo with respect and calls him Mr. Arthur. However, he does not talk about him often as he respects Boo's privacy, and repeatedly asks Jem and Scout to leave him alone and not ridicule him. When the children tried to give Boo a note and Atticus caught them, he told them sternly and for about the last time to leave the man in peace and to stop playing the game that he had seen them playing.
The children at the beginning of the book had a completely different image of Boo, as can readily be seen by the description of Boo that Jem gave Dill at the beginning: six and a half feet tall, bloodstained hands, a long jagged scar, etc. They saw him as a monster and anything bad that happened was blamed on him. For example, Walter Cunningham said that he almost died when he ate some pecan nuts from the ground - understandable as they had probably been lying there for a few days. However, his explanation was that Boo Radley had poisoned them and put them over the fence. Scout wasn't comfortable playing the game that Jem made up about the Radleys for two reasons: one, that she thought that Atticus wouldn't like it, and the second, that the time she rolled into the Radley front yard, she heard someone laughing softly inside the house.
Stephanie Crawford, the town's main gossip spreader, adds to the portrayal of Boo as a 'bogey man' and gives the children first-hand information. She even told Maudie Atkinson once that she woke up in the middle of the night and found him staring at her. She shares the children's views when they are younger, but during the book they mature and see Boo for what he is, while she sticks with her judgement of him and refuses to revise her opinion of him.
Jem at first glamorises Boo Radley as a character to frighten children with, but during the book, he finds small clues and hints that means he is constantly having the reassess Boo's character. The trousers hanging neatly on the fence, the presents in the tree and Boo placing a blanket on Scout's shoulders to keep her warm all reveal the nature of Boo, which is that of a caring, kind-hearted man who likes to observe 'his' children at play, hence the laughter when Scout rolled into the front yard in the tyre. Jem comes to realise this and cries when Nathan Radley cements the knothole in the tree, thereby cutting off the small form of communication that Boo had.
Boo Radley is an important character in the book, as he reveals other people's characters in the way that they perceive him; the children believed rumours that they heard and loved to dramatise it, but then matured enough to tell the difference between rumours and the truth, Stephanie Crawford loves to spread gossip, Maudie Atkinson and Atticus rely on evidence and their own shrewd natures. However, Boo becomes more important in Part 2 of 'To Kill A Mockingbird'.