Who in the novel To Kill a Mockingbird is invisible? Why? To whom are they unseen?

Of course, no one in the novel To Kill a
Mockingbird is literally invisible, though Boo Radley seems that way to Jem
and Scout and Dill. Once Dill arrives to spend the summer in Maycomb, the children set
their sights on a single objective: getting a look at Boo. They come close one night,
when Boo's shadow is seen while the children cower outside the Radley's back porch.
Scout misses out on another opportunity on the night when Miss Maudie's house burns:
When she discovers a blanket draped across her shoulders, Atticus explains that it must
have been Boo who put it there.

Of course, people have
caught glimpses of Boo now and then. Atticus has apparently seem him on occasion, as has
the town doctor, who has treated Boo. But Boo remains invisible until the Halloween
night when Bob Ewell attacks Jem and Scout, and Boo reveals himself in order to rescue
the children from their attacker. Scout finally sees him at last, standing in the
shadows of the Finch house. Her dream comes true as she looks at her protector for the
first time, and when it comes time for Boo to leave, she proudly walks him back to his
house. She would never see Boo again.