There are many memorable characters which we have have read about, take snape, or gollum/smeagle for instance. Character's which are so very memorable and well written that we can't forget them or get them out of our head, how about sharing some of them? Evil or good, greedy or mysterious, however they are.
I'd agree with you Astor - in fact, I think you could argue that all the characters in To Kill a Mockingbird are incredibly memorable. They're all so realistic - I particularly like Aunt Alexandra as she's so changed by the end of the novel.
How about the mysterious Boo Radley from To Kill a Mockingbird? The strange reclusive neighbour of the Finch children about whom they conjure up all sorts of wild fancies?
As the reader, we only get to meet him toward the end of the book after he saves the childrens lives from a knife weilding maniac. But throughout, his presence is felt and by the time he does make an appearance, we are all wondering just what he is really like.