Can the AI figure out the characters and their relationships from a book I already wrote?

Yes, in one click. Open the book, click "Codex" above the chapter list, and in the Characters tab press "Extract from manuscript". The AI reads your finished chapters and drafts a card for every character who acts or speaks — including profession, backstory hints, desires and struggles where the text establishes them — AND maps the relationships it finds between them (who is whose wife, boss, friend, enemy). It works on imported manuscripts too. Anything you filled in yourself is never overwritten: extraction only adds new characters, fills empty fields and draws relationships that aren't drawn yet.