Obviously, Harry himself as the son of Atlas. Then there could be the original characters like Dumbledore, who might play a role in guiding Harry about his new identity. Hermione would still be there, probably helping Harry research about Atlas. And Ron, as Harry's friend, would be involved in his adventures.
There could be new characters introduced as well. Maybe some descendants of the friends or enemies of Atlas in the wizarding world. For instance, a character who claims to be a protector of Atlas's legacy and tries to help Harry. Or an antagonist who wants to use Harry's connection to Atlas for their own evil purposes. There might also be some mythical beings related to Atlas's story that appear in the fanfic.
Potential characters could include Harry's parents in a different light. Since Harry is the son of Atlas, his mother Lily might have had some special connection to Atlas as well. His father James could also have been involved in some way that wasn't known before. And then there are the Hogwarts professors. Snape might be even more suspicious of Harry with this new identity, while McGonagall could be trying to keep things in order at Hogwarts while Harry deals with his newfound heritage.
Obviously, Harry Potter himself. Then there could be the dragons, perhaps an old and wise dragon that acts as Harry's mentor. Hagrid might also play an important role as he has a connection with magical creatures and could help Harry understand his new identity.
There could be some sort of primordial council or group of elders in this fanfic. They would be the ones who have knowledge about the primordials and their descendants. Harry would interact with them, and they could either be his allies or his antagonists. Then there are the regular students at Hogwarts. Some might be jealous of Harry's new powers, while others might be in awe. And we can't forget about the magical creatures, like centaurs or house - elves, who might also have a different view of Harry due to his primordial connection.
The main character is Harry Potter. His friends Hermione and Ron are likely to be main characters as well. There may be some antagonists from the Titan realm who oppose Harry because of his potential power. There could also be some other Hogwarts students who get involved in Harry's journey to discover his true identity as the son of Atlas. And perhaps an old wizard who has knowledge about the connection between the wizarding world and the Titans and helps Harry along the way.
Obviously, Harry Potter himself as the main character. Then there would be the members of the Black family, like Bellatrix Lestrange who could have a different attitude towards Harry. And of course, Sirius Black would play a different role as Harry's family member.
Well, it could be about Harry having powers related to Atlas. Maybe he can control the sky or something like that. It might also involve him having a different destiny compared to the original Harry Potter story.
Hermione would definitely be there. She would be intrigued by Harry's new heritage and would probably try to research all about Ra in the Hogwarts library. Ron too, would be by Harry's side, though he might be a bit more skeptical at first about this whole Ra business.
In this fanfiction, Voldemort could also be involved in a different way. Since Harry is now the son of the Demon King, Voldemort might see him as a threat or a potential ally. Dumbledore would surely play a role too. He might be the one to guide Harry or try to keep him from fully embracing his demonic side. And there could be some magical creatures that are associated with the demonic realm, like imps or minor demons that could be characters in their own right.
Obviously Naruto is one of the main characters. Since it's about him being the son of Atlas, he'll be at the center of the story. Another main character could be a mentor figure who helps Naruto understand and control his powers related to Atlas.
A new young teacher at Hogwarts. She could be a prodigy in Transfiguration, having mastered some really advanced spells at a young age. She brings a fresh perspective to teaching and has a bit of a rivalry with McGonagall in the fanfic. She might also have a mysterious past that is gradually revealed throughout the story.
Obviously, Madam Pomfrey is a key character as she runs the hospital wing. Harry, Ron, and Hermione could also be characters, perhaps they visit a friend who is ill. There could also be some minor characters like Neville who might get injured during a Herbology class and end up in the hospital wing.