Since the definition of an eccentric person is one
"who has an unusual, peculiar, or odd personality... or behavior pattern,"
or one who deviates from customary practices, it would seem that the word fits Boo Radley perfectly. Very little is known about Boo other than the rumors that are spread about him, but we do know that he only comes out of his house at night; that virtually no one has seen him for nearly 30 years (aside from when he stabbed his father with a pair of scissors); and that he secretly hides items in a tree for Jem and Scout. Remaining a self-imposed recluse within his own house is considered odd by most people, and these facts, along with his past erratic behavior, solidifies Boo as one of the most eccentric men in Maycomb.