It depends on where in the book you want to reference. If you want to reference the kids' stories and the town gossip, Boo is a mean man, over 6 feet tall. His hands are blood-stained from eating raw squirrels and cats. His eyes bug out and he drools all the time. Anything on the Radley lot is poisonous and Boo's breath causes the unexpected frost that may come upon plants at night. Boo is a thing of legend, gossip and town lore.
By the end of the novel, the true personality of Boo is revealed. He is a kind man--he leaves gifts for Jem and Scout in the tree, puts a blanket on Scout the night MIss Maudie's house burns down, and save their lives from Bob Ewell. Boo may also be seen as a bit lonely. He looks just like a regular man, though a bit more pale and skinny. He is shy and quiet and doesn't like to be the center of attention.
In reality Boo is a kind person who has just been isolated. He left gifts for the children in order to communicate with them and he also saves Scout and Jem from getting killed. He looks like a normal person except a little bit more skinnier and he does not like to be at the center of attention. He is a mockingbird and he has a kind heart.