Nathan Radley is Boo Radley's older brother, and he continues their father's tradition of keeping Boo isolated from the community. Boo knows, however, that Jem and Scout have taken a great interest in him. He reaches out to them by leaving the two children small gifts in the hollow a tree.
When Nathan realizes what is going on, he fills up the knothole with cement. He explains to Jem when confronted that he did it because the tree is dying. However, the tree is healthy. It is evident that Nathan wanted to cut off the communication between his brother and the children.
This help makes it clear that Boo is more sinned against than sinning. He doesn't wish to be an isolated figure that the children fear. Miss Maudie explains to Scout that Boo's father had the wrong kind of religious conversion experience, one that left him judgmental and punitive and caused him to isolate his family. Nathan is continuing along that path.
Luckily for the children, Boo's spirit can't be quenched, and he continues to take an interest in and look out for them. He saves their lives when Bob Ewell tries to kill them in retaliation for Atticus humiliating him at Tom Robinson's trial.