Scout realizes that the man who rescued her and Jem is Boo Radley, how is his actual appearance different to what they imagined?

In Chapter 29, with the description of his hair as "feathery," Boo is immediately identified with the "mockingbird," especially with his slight appearance and fluttery hand movements. He has finally become a real person, completing the progression from monster to human; meanwhile, Mr. Ewell's evilness has turned him into a human monster, whose bristling facial stubble felt by Scout suggests an animal-like appearance. When Scout addresses Boo directly, she makes her final step into the beginnings of maturity, leaving her childhood imaginary tales behind. As a mature young girl, she recognizes Boo as a real person, and treats him as such.