what is the reader discovering about arthur radley

This depends on what chapter you are on. We learn more about him as the novel progresses. Can you be more specific?

Sarah,

Arthur "Boo" Radley doesn't like going out as much as "Jem", Charles B. Harris, and Scout. The children write a letter to Arthur and try to make him come out. Atticus soon finds out that the children are tormenting Mr. Radley and constantly asks them to stop. Arthur has a right for his own privacy.

Yeah, I agree Allie.