a closed book
Someone or something that is difficult to know or understand. She's such a private person that I don't actually know very much about her. I guess I married a closed book. I'm a scientist through and through, so art has always been a closed book to me.
closed book, a
A secret, mystery, or puzzle, as in I can't figure her out; she's a closed book to me. This term alludes to information one can't obtain or comprehend (because the book is closed). [Early 1900s]