There could be Sasuke. Since Naruto's situation changes, Sasuke might play a different role in this fanfic, either as an ally or a rival in a new way. Another might be Kakashi. He could be the one guiding Naruto through his discovery of being the son of chaos.
In this fanfic, aside from Naruto, I think Hinata could be a main character. Given her feelings for Naruto, she may be deeply involved in his journey as the son of chaos. Also, Pain could be a significant character. His ideology might clash with Naruto's new - found identity and powers, leading to some intense confrontations.
Samul, of course, since he's Naruto's supposed father in this fanfic. He could be a powerful ninja who had some great influence in the past. Another character could be Naruto's mother, who might have a different story now considering her relationship with Samul. Also, characters like Jiraiya could still play an important role. He might be the one to guide Naruto in understanding his new identity as Samul's son and help him train to face the challenges that come with it.
Another main character could be Kakashi. As Naruto's mentor, he would have to guide Naruto in understanding and using his powers related to being Raizen's son. He might also be the one to protect Naruto from those who would seek to harm him because of his new identity.
Well, in addition to Naruto, Gaia herself could be considered a main character in a sense, even if she might not be a physical presence all the time. If the fanfiction delves into Naruto's heritage, there could be other characters related to Gaia's past or other 'sons' or 'daughters' of Gaia in the Naruto world. And of course, Naruto's friends like Sakura would also be important. Her reaction to Naruto's new identity could range from shock to being supportive, and she might be involved in Naruto's new adventures related to his connection to Gaia.
Naruto would likely still be one of the main characters. Since it's a Naruto - related fanfic, he's bound to play a central role, probably with his new chaos - influenced abilities or situation.
Gohan himself could be a main character in some form. Maybe he exists in Naruto's memories or as a spirit guide. Another potential main character could be Sakura. She might be interested in Naruto's new powers and try to help him understand and control them. And of course, there could be new characters introduced from Gohan's world that interact with the Naruto characters.
Sasuke could be one of the main characters. He might be suspicious of Naruto's new powers and how they change Naruto. Their relationship could become more complex as Sasuke tries to understand if Naruto is still the same person he knew.
The main characters in a 'naruto heir of chaos fanfic' might be the heir of chaos, of course. They could be a young ninja with a troubled past, perhaps an orphan who discovers their unique heritage. Naruto would likely be involved too. Maybe there are also some other well - known Naruto characters like Sasuke or Sakura. Sasuke could be suspicious of the heir of chaos and see them as a threat to his own goals, while Sakura might try to understand the heir's motives and help them fit into the ninja world if she believes they are not truly malicious.
Sasuke could be one of the main characters. He might be initially skeptical or even hostile towards Naruto's mermaid side, but as the story unfolds, he could come to understand and support Naruto. Another character could be Sakura. She could use her medical ninjutsu skills to help Naruto if he gets injured during his underwater adventures.
Kushina could be an important character even though she's dead in the original story. In this fanfic, her spirit or memories could play a role in Naruto's upbringing. For example, Minato could tell Naruto stories about Kushina's feisty personality and her love for the village.
There could be some original Asian - inspired characters. For example, a young martial artist from a small village in China who Naruto takes under his wing. This character could be eager to learn from Naruto and would be one of the main focuses as he grows under Naruto's guidance.