One characteristic of Romanticism is that, since Romantic thinkers placed great value on the individual self, they also exalted the deeds performed by the social outcasts, those who were misjudged by society. In To Kill a Mockingbird, we particularly see Harper Lee employ this Romantic characteristic when she promotes the actions of Arthur (Boo) Radley.
The Radleys are considered social outcasts of Maycomb because they keep to themselves, never attend church, and keep their doors and windows shut on Sundays. Arthur is especially treated as an outcast because he never leaves his house. Since he never leaves his house, the townspeople have developed all kinds of rumors and myths about him that lead children to feel afraid of him. As the story progresses, however, Arthur is revealed to be a caring and benevolent person and one of the novel's major heroes.
Arthur demonstrates his caring and benevolent nature by leaving the children gifts, sewing Jem's torn trousers, and covering Scout up with a blanket on the cold winter's night when Miss Maudie's house burns down. His most heroic moment is when he rescues Jem and Scout from Bob Ewell, who is bent on getting revenge on Atticus. It is at the moment when Arthur becomes the story's greatest hero that Harper Lee employs the Romantic characteristic of exalting society's outcasts.