Although Jem and Scout have never seen Boo Radley, Jem offers a description of the "malevolent phantom" early in the novel.
Boo was about six-and-a-half feet tall, judging from his tracks... There was a long, jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time.
Scout discovered later that none of this was true when she met him face-to-face for the first time on that fateful Halloween night. When he stepped from the corner of Jem's room, Scout saw that he was painfully thin: he had a "thin frame," his "cheeks were thin to hollowness," and his hair "was dead and thin, almost feathery on the top of his head." In one way, Boo lived up to his name: He was white as a ghost. He had
... sickly white hands that had never seen the sun... His face was as white as his hands...
Boo's "mouth was wide," and his eyes were gray, "so colorless I thought he was blind." He coughed often and shuffled his feet when he walked, but when Scout took his hand, she found that it was "surprisingly warm for its whiteness."