Old Mr. Radley was the father of Arthur Jr. (Boo) and Nathan Radley, who lived several houses to the south of the Finch house in Harper Lee's novel To Kill a Mockingbird. Jem and Scout would see Mr. Radley walking down the street each morning; he would pass by the Finch house at 11:30 and return by noon, usually with a "brown paper bag that the neighborhood assumed contained the family groceries." He was not a friendly man, and when Scout would say "Good morning, sir," he would only cough. Mr. Radley was
... a thin leathery man with colorless eyes, so colorless they did not reflect light. His cheekbones were sharp and his mouth was wide, with a thin upper lip and a full lower lip... he was so upright he took the word of God as his only law, and... Mr. Radley's posture was ramrod straight.
When Mr. Radley died, his son, Nathan, returned from Pensacola. He would take his father's place as the head of the household, and the watcher over Boo.
The above answer gives the only physical description of Mr. Radley. He is such a minor character that not much is said about him. Moreover, all of these descriptions are recollections from the past. So, the reader is not certain if these descriptions are accurate. Here is the full text:
My memory came alive to see Mrs. Radley occasionally open the front door, walk to the edge of the porch, and pour water on her cannas. But every day Jem and I would see Mr. Radley walking to and from town. He was a thin leathery man with colorless eyes, so colorless they did not reflect light. His cheekbones were sharp and his mouth was wide, with a thin upper lip and a full lower lip. Miss Stephanie Crawford said he was so upright he took the word of God as his only law, and we believed her, because Mr. Radley’s posture was ramrod straight.
To help us in our endeavor to gain a description of Mr. Radley, we can also examine little statements here and there. In one instance, Miss Maudie tells Scout that Mr. Radley was very strict. He was a religious fanatic. He was a "foot-washing baptist." The title meant that he was extremely austere and severe. This partially helped to explain why Boo Radley is a tragic figure.
Here is what Miss Maudie says about foot-washers:
Apparently deciding that it was easier to define primitive baptistry than closed communion, Miss Maudie said: “Foot-washers believe anything that’s pleasure is a sin. Did you know some of ‘em came out of the woods one Saturday and passed by this place and told me me and my flowers were going to hell?”