There could be some new vigilantes. Maybe a young hero who looks up to Batman and tries to help him. Another possibility is a new villain from outside Gotham who has a unique set of skills and motives.
I'm not sure as I don't have specific information about 'the new soul story'. It could have characters like a young soul seeker, an old wise soul guide, or perhaps a group of souls on a shared journey.
😋I recommend the Kamen Rider's Dark Rider: Light's Hunt for Novelty. The main characters of this book, the Dark Knight and the Light Hunter, will travel to different knight worlds together and embark on an adventure full of curiosity and darkness. I hope you like this fairy's recommendation. Muah ~😗
Obviously, the Arkham Knight is the main focus. However, we can't rule out characters like Harley Quinn. She has been a significant part of the Arkham series before. Also, Robin or Nightwing could potentially make an appearance, depending on how the story unfolds. And there may be some new villains or anti - heroes that are specific to this 'new knightmare story' which will add more depth and complexity to the plot.
Since it's a RWBY fanfic, there are likely to be some of the main RWBY characters like Ruby, Weiss, Blake, and Yang. As for the knight, they would be a main character too. There might also be other characters from the RWBY universe that the knight interacts with, like Ozpin or Qrow.
The Masked Knight's Spirit Knight's Infinite Soul Form first appeared in episode 34. This was the final form of the main character in the Japanese drama " Ghost the Kamen Rider." Infinite Soul was transformed by using Ghost Drive and Infinite Ghost Eye Soul. It used the Eye Gun Saber and Sunglass Saw Blade as its weapons, and strengthened its power with human emotions and infinite possibilities. The Unlimited Soul Form appeared in the 33rd episode as a glowing image and officially appeared in the 34th episode.
A new villain from a foreign land. Maybe a warlord type who has heard of Gotham's chaos and sees it as an opportunity to expand his empire. He brings his own set of minions and a different kind of warfare to Gotham, forcing the heroes to adapt to new fighting styles and strategies.