It's possible that villains from both universes could also play a part. In Naruto, Orochimaru might be involved somehow, maybe he's the one who spreads false rumors about Naruto to the RWBY characters. In RWBY, Salem could be involved. She might see Naruto as a threat to her plans and manipulate the RWBY characters into betraying him.
Obviously, Naruto would be one of the main characters. He's the one being betrayed. From RWBY, Ruby, Weiss, Blake, and Yang are likely to be involved. They are the central characters in RWBY, and if there's a betrayal of an outsider like Naruto, they would probably play a significant role in it.
In addition to Naruto, there could be other supporting characters from Naruto's world like Sasuke or Kakashi who might be affected by his betrayal. On the RWBY side, characters like Ozpin or Qrow could also be involved. Ozpin because he's a leader figure and Qrow as he often gets involved in the main events. Their actions could contribute to the betrayal or the events following it.
Well, obviously Naruto himself is the central character. Sasuke could be involved as he has a complex relationship with Naruto. In a crossover, if Naruto is betrayed and ends up in the Dragon Ball universe, Vegeta might be a character he interacts with. Vegeta could initially see Naruto as a weakling but later come to respect his skills. Another character could be Luffy from One Piece. If Naruto is betrayed and crosses over to the One Piece world, Luffy would likely befriend him and help him out, seeing Naruto's situation as similar to his own crewmates' struggles.
Cinder Fall could be a likely betrayer. She's already a villain in the original series, and in a fanfiction, she could pretend to be an ally to one of the RWBY members only to betray them at a crucial moment, perhaps leading them into a trap set by Salem.
Obviously, Percy Jackson is the central character. The Olympians such as Zeus, Poseidon, and Athena are likely to be involved as the ones doing the betraying. Other demigods like Annabeth Chase could also be part of the story. She might be torn between her loyalty to Percy and her respect for the Olympians. There could also be minor gods and goddesses who either side with Percy or the Olympians, depending on their own motives and relationships. For example, Hades might see an opportunity to gain more power if Percy is out of the picture, or he could help Percy out of a sense of fairness.
Obviously Naruto himself is the central character as it's about his hibernation. His friends like Sasuke and Sakura are also likely to be involved. They would probably be the ones most concerned about his condition and try to figure out what happened.
The village elders are often depicted as the ones involved in Naruto's betrayal. They might see him as a threat to their power or the stability of the village. Also, some fanfictions might involve Danzo. He has always been a bit of a shadowy figure in the Naruto series, and his underhanded ways could easily be written into a plot where he is behind Naruto's false accusations and subsequent banishment.
Well, obviously the main RWBY characters like Ruby, Weiss, Blake and Yang are likely to be involved as they are the central figures in RWBY.
Well, Naruto for sure. His immortality gives him a unique place in the story. From RWBY, Yang Xiao Long could be a main character. Her brash and confident fighting style would create an interesting dynamic with Naruto. Blake Belladonna, with her mysterious past and cat - like abilities, might also be a central figure. Her more reserved nature compared to Naruto's open - heartedness could lead to some great character development moments. And of course, Ruby and Weiss as mentioned before, would also be important in shaping the story around an immortal Naruto in the RWBY world.
Obviously Naruto would be one of the main characters as the title implies he is being trained. And then, the RWBY characters like Ruby, Weiss, Blake and Yang would also be main characters since they are the ones doing the training.
It could also involve Naruto's future family in some time - travel or alternate reality scenarios. For example, if Naruto from the future travels back in time and his future family members come with him. They might have been corrupted in the future and their actions in the past, like betraying Naruto, could be part of a chain of events that led to that corruption. This would create a complex web of relationships and a unique take on the 'betrayed by family' concept.
Naruto is the primary one for sure. But there could be other characters like Jiraiya. Jiraiya is known for his knowledge of various things in the Naruto universe, and he could play a role in teaching Naruto about demon breeding. Additionally, some of the demon characters themselves could be considered main characters in a sense. They would have their own personalities and goals, and their interactions with Naruto would be a major part of the story. For example, a powerful demon that Naruto is trying to breed might have a complex backstory and motives that drive the plot.