There might also be some original characters created specifically for this fanfiction. For example, a character who was once an outcast in the Gremory family but becomes Naruto's closest ally. Or a powerful enemy who has a vendetta against the Gremory family and sees Naruto as an easy target since he's new to the family.
It could be that some of the neglected characters from Naruto, like Rock Lee for instance, are made main characters in this fanfiction and are paired with a Gremory character. Maybe a Gremory character who also has been somewhat overlooked in their own series. This would give a new perspective and new storylines to explore.
Obviously, Harry Potter himself would be the main character. Hermione and Ron would also be important as they are Harry's closest friends and would be involved in his journey as the heir.
In this fanfiction, the plot could also center around Naruto trying to understand the Gremory family's history and secrets. As he delves deeper, he discovers things that could change the future of the family. He might also have to deal with enemies from both the Naruto universe and the Gremory universe who are out to stop him from becoming a successful heir. He would need to use his quick thinking and combat skills to protect himself and the family.
Obviously, Harry Potter would be one of the main characters. He brings his own story, skills, and personality from the Harry Potter series. And Rias Gremory would also be a main character as she is from her own universe. Her leadership and unique abilities would play a major role. Hermione and Ron could also potentially be main characters as they are Harry's closest friends. In the Rias Gremory side, her peerage members like Akeno or Koneko might also be important characters in the fanfic.
There could also be some well - known Naruto characters who play important roles in the story. For instance, Naruto or Sasuke could be involved, either as mentors or as characters who are somehow related to the events involving the heir. Maybe Naruto helps the heir come to terms with the deaths they are associated with, or Sasuke has some knowledge about the secrets behind the deaths that the heir needs to know.
Obviously, the heir of Madara would be a main character. There might also be other characters from the original Naruto series, like Naruto himself. Naruto could be in opposition to the heir, seeing them as a threat to the peace he has worked so hard to achieve.
Obviously Naruto would be one of the main characters. There might also be his usual friends like Sasuke and Sakura. But since it's a fanfiction about him being the heir of chaos, there could be new characters introduced who are related to this chaos aspect, like some ancient beings or a secret clan.
Well, Naruto is obviously one of the main characters as it's about him being the heir of darkness. Sasuke could also be a major character as his relationship with Naruto is always a central aspect in Naruto - related stories. Sakura might also play an important role, perhaps being one of the first to notice the changes in Naruto due to his new status.
Obviously Naruto would be one of the main characters since it's about him being the heir of Kaguya. Sasuke could also be a main character as Naruto's rival and friend. Sakura might be involved too, as she is part of Naruto's team. And there could be other characters like Kakashi, who would play a role in guiding Naruto as he discovers his new identity. There might also be some new characters introduced that are related to Kaguya's past, who either help or hinder Naruto on his journey.
Obviously Naruto would be one of the main characters since it's about him being the heir to the Uzumaki. Sakura and Sasuke might also be important characters as they are Naruto's friends and comrades. There could also be some original characters created specifically for the fanfiction, like an old Uzumaki sage who guides Naruto on his journey as the heir.