Their next door neighbour's son "Boo" Radley is the "phantom" in this story simply because he never comes out. Although he is a young man, he lives at home under the iron thumb of his father since he is being punished for various "crimes" he committed when he was an adolescent. He had stolen a car and gone on a joyride along with some of his buddies; upon another occasion he had locked up a school official in an outhouse and wouldn't let him out. This was enough for a local judge to order that the troublemakers be sent off to a vocational school as part of their probation. Arthur's (Boo's) father resisted, and promised to keep his boy at home - out of sight, out of mind - and he lived up to his word.
As Tom Robinsoon, "Boo" is the brunt of Maycomb's suppositions regarding him and is the victim of their prejudice. According to rumours, he is a night stalker and peeping Tom, feeds off raw squirrels, and is potentially violent. The Finch children even act out "the scissors scene" on their front porch (in which Boo allegedly stabs his father in the leg, then calmly continues cutting up a newspaper) until Atticus puts a stop to their mime.
As the story unfolds, Boo manages to befriend the Finch children despite his father's interdition to go out. Later he even saves them from Mr. Ewell's attack with a knife one Halloween night. Jem is injured, but Mr. Ewell is killed when Boo fends him off.
Boo represents anyone who is marginalized and misunderstood by society; thus the message in the story concerns not just racism but encompasses prejudice of any kind. The children's contact with Boo illustrates that people are not always what they seem to be, and that everyone needs acceptance and to "belong," despite former mistakes they may have made. It is ironic that it is Boo - the hermit and the misfit, the fragilized citizen of Maycomb - who arrives on the scene and has the courage to defend Jem and Scout from Ewell's attack.