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."
Without having met Boo, Scout, Jem, and Dill imagine Boo as a kind of monster who is "about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch." Jem imagines that Boo has a long scar on his face and bloodstained hands.
Interestingly, when Scout meets Boo after Boo has saved her life and Jem's, she offers a very scant physical description of Boo. He has a white face and white hands, hollow cheeks, gray eyes that are almost "colorless," and hair that is "dead and thin, almost feathery."Scout describes the way Boo moves—uncertainly—and says that Boo has a cough and takes a handkerchief to his mouth. Boo speaks very little and communicates with Scout by squeezing her hand to let her know he wants to go home. He only says, "Will you take me home?" as a scared child would. She takes him home and mentions that he has to stoop to do so, indicating that he is taller than she is. She says she never saw him again. The reader knows little about how Boo actually looks, but can tell from his motions and actions that he is an exceedingly gentle and scared soul.