What a co-author is for
A good co-author does the work you find slow and leaves the decisions to you. Here that means you set the idea, the tone and the direction, and the AI drafts the chapters. Before it starts, it asks up to three questions about what your idea leaves open, so the first draft is already close to what you meant. Answer the ones that matter and skip the rest.
Your voice, or one of theirs
No Writer is chosen on this page, because a co-author should sound like you: a plain voice you shape with your answers and your edits. If you would rather borrow a style, Spicy writes boldly, Pyra slowly and warmly, and Onyx in a darker key. Set the heat from none to the highest levels for readers 18 and over, and the chapter length from about 500 words to several thousand. Writing happens in the full writing app after a free sign-up, where you can ask for a rewrite of any scene, and every chapter remembers the last.