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Boo Radely is much like a Mockingbird in that he innocent. He meant only to do good things for Scout and Jem. Boo's rescue of Scout and Jem was selfless. Casting attention on him would only harm him. Boo needs his privacy and telling the truth, in this case, would cause more harm than good.
So, Boo is essentially a "mockingbird-figure" in the book.