The book tries to flesh out the movie.
The Star Wars: The Rise of Skywalker novelization reveals Rey's father is actually a failed Palpatine clone. One of the biggest (and most controversial) reveals in the film is that Rey is the Emperor's granddaughter, descending from ultimate evil. While Rise of Skywalker director J.J. Abrams and co-writer Chris Terrio have done their best to explain why this plot twist is compelling, some viewers took issue with how it seemingly retconned The Last Jedi's stance that Rey came from nowhere and her parents were nobodies. The Rise of Skywalker tries to "from a certain point of view" its way around this, but it didn't work for everyone.
Besides arguably clashing with the themes and developments of Last Jedi, Rey Palpatine obviously had huge implications within the larger Star Wars canon. At the time of The Rise of Skywalker's release, the information presented made it appear as if Palpatine had been intimate with an unknown woman, giving birth to a son. For the past few months, fans have speculated who Rey's grandmother could be, but now it's been confirmed she does not have one.
During the scene in the Rise of Skywalker novelization when Rey is feigning taking part in the Sith Ritual on Exegol to trick Palpatine (right before she passes the lightsaber to Ben), she has visions of her grandfather's past. This passage in the book reveals after Return of the Jedi, Palpatine "thrust his consciousness" into a clone body. However, "the transfer was imperfect" and members of the Sith Eternal worked tirelessly to engineer a new vessel for Palpatine's essence. One of these attempts is described as "a useless, powerless failure" who was "a not-quite-identical clone." While this body wasn't fit to house Palpatine's power, it was still able to live and eventually became Rey's father.
Back in January, Rise of Skywalker editor Maryann Brandon revealed they cut additional Palpatine backstory in an effort to condense the film, and it's possible this information is what she was referring to. Brandon had mentioned that initially the movie got more in depth into "what was keeping Palpatine alive." And it's been confirmed the Rise of Skywalker scripted included a line about Palpatine being a clone. While it's understandable why the Rise of Skywalker creative team would want to keep things moving and economically find a way around the topic of Palpatine's return, a case can be made it would have been good to include some of this in the film. It may have cleared up questions some people had. Palpatine's role in Rise of Skywalker received criticism in some circles for coming across as sudden and tacked on within the context of the sequel trilogy. His past between Return of the Jedi and The Force Awakens could have been fleshed out.
Anyone still care about this trash?