Nathan Radley is the brother of the mysterious Boo Radley in Harper Lee's novel To Kill a Mockingbird. After Boo's father dies, Nathan takes over as head of the household and, presumably, as the protector of Boo. Very little is known about "Mr. Nathan," and he is only mentioned a few times in the story. His most prominent appearance comes when he cements up the knothole in the tree--the secret hiding place where Jem and Scout communicate with Boo. He also fires a shotgun at the kids one night, and helps out a bit on the night of the big fire. He is a minor character, serving only as a connection to Boo.
Nathan is the brother who probably suffered more abuse than Boo and this may be why he wanted to sheild his brother from the outside world. Older siblings tend to be more protective and imitate the actions of their parents., hence the mysterious ways of both Boo and Nathan. Nathan's part was minor due to him only having to show what he was left to carry on the actions of the parents.
Nathan is Boo's older brother and he represents the vileness of life. While he had grown up in a less than stellar home, he allowed his own problems to make him a bitter person in the end. He unwillingly becomes the head of the house and it is apparent that he dislikes Boo. His life of misery comes about because he deals with the awful hand that he feels like dealt him. He is your typical "misery loves company" character. He's not so much a protector for Boo, but a person who is bitter about his role in life.
Boo, on the other hand, is a strange person, but he is like that "mockingbird"....only there to bring happiness to others. His odd behavior hurts his family at times, but he is a savior to those on the outside.
In my humble opinion, Nathan and Boo represent the affects of negative and evil influences of human nature. Nathan is the older brother and therefore had to live in the house with an abusive father longer than Boo. In the absense of that "father," Nathan becomes that to Boo.
The differences between them are illustrated in Boo's generosity to Scout and Jem by leaving them treats in the knot hole; whereas, Nathan is very territorial and desires no contact with outsiders. His cruelty is demonstrated in plugging the hole, thereby, cutting off Boo's only outlet to the outside world.
The contrast in their characters is demonstrated when Boo leaves what is comfortable in order to help the children at the end of the story. Contrast this with the image the reader has of Nathan as obstinate and cruel and you see the differences in human nature represented within the same family.
The link attached below has a pretty good synopsis of the story and examining the characters in detail. Hope this helps.