HUMAN: Boo was so afraid of the outside world and how people would react. The people would shun him. This relates so well to this generations issue of bullying. Afraid to even leave your home.
Daring: Boo hasn't been in contact with any of the outside world for years. But even though he barely knew scout and Jem, he knew he couldn't just let them get hurt.
Boo may not have been the the protagonist of TKaM. But he was surly the hero of her. And Boo is still one of the most remembered story characters to this day.