At the beginning of the novel, Jem, Dill, and Scout are fascinated with their reclusive neighbor Boo Radley. After hearing the various rumors and legends surrounding Boo, the children become interested and make several attempts to look at him. Since Boo never leaves his house, the children become curious and begin to obsess over the identity of Boo. Jem describes Boo as a grotesque monster and Scout refers to Boo as the "malevolent phantom." Dill comes up with elaborate plans and schemes to make contact with Boo, and Scout fears that Boo will harm them if he sees them reenacting his life story in their front yard. The rumors surrounding Boo pique the children's interest, and their curiosity gets the best of them. They are also forbidden from bothering Boo Radley, which only heightens their willingness to see him and fuels their active imaginations. Boo represents the "unknown" and the children are fascinated with understanding who he actually is.
The answer to this questions is certainly a bit subjective.
I think it is fair to say, however, that Boo represented an element of excitement. In a town where everyone seems to know everyone, he remained a mystery. The stories about him were certainly enough the stir the imagination of children whiling away summer days in the lazy heat.
Check out E-notes character analysis for more details on Arthur (Boo) Radley as well as the three children.
because he's always in the house no one has seen him