I think that's true of real life - we don't ever know anyone completely. Secrets are very important to creating a narrative work that's believable. The characters come into that world with secrets, as happens to all of us. As honest as we try to be in our relationships, we can never completely know someone. From a narrative perspective it's very important and pleasing - you want to have those secrets there. The secret is an essential part of the creation of the novel.