Sasuke might also be a main character. His relationship with Naruto is so fundamental to the Naruto universe that in a fanfiction about Naruto being reborn, Sasuke would likely still play a major role, perhaps as an ally or rival in a different way than before.
Kakashi could be a main character too. He was Naruto's teacher and mentor in the original story. In a 'naruto is reborn' scenario, Kakashi might notice the differences in Naruto and play a key part in guiding him through his new life, or be surprised by how Naruto already seems to know so much more.
Obviously Naruto is one of the main characters as the title implies. Since it's a 'reborn' story, his new self with all the changes is central to the plot.
Well, aside from Naruto and Issei, there could be other important characters. Rias Gremory, who is a devil princess in DxD, might play a significant role. She could be curious about Naruto and his powers, or she might even see him as a potential ally or threat. Also, characters like Akeno Himejima, Kiba Yuuto, and others from the DxD series could be part of the story and interact with the reborn Naruto in different ways.
Mainly, it's the reborn Naruto taking on the role of Kiba. Akamaru is an essential character as they work together in combat and daily life. Kiba's sensei, like Kurenai, will also be an important figure. She will be training this new 'Kiba' who has Naruto's essence inside. And then there are the other Konoha ninjas. They will interact with the reborn Naruto as Kiba, and their reactions to the different behavior and skills of this 'Kiba' will be a significant part of the story.
Obviously, Harry (reborn as a character in the Naruto world) is a main character. Naruto himself is also a key figure as Harry will interact with him. Sasuke could also be important as he is a central character in the Naruto series and his relationship with Naruto and any new character like Harry will play a role in the story.
Obviously, Naruto would be one of the main characters since the story is about him being reborn as a devil. There might also be some original devil characters created specifically for this fanfiction that interact with Naruto. And perhaps some of Naruto's old friends from the original series might also play important roles, like Sasuke or Sakura, even if their relationships with Naruto are now different due to his new form.
Obviously Naruto would be the main character since it's about him being reborn as a Pokemon fan. His Pokemon partners could also be considered main characters. For example, a Pikachu that he befriends early on and they go on adventures together.
Definitely Shikamaru's friends like Choji. Naruto in Shikamaru's body would still have that bond with Choji, but it would be different. They might have conversations about food and friendship in a new light. Also, Ino would be a major character to interact with. Their relationship could change as Naruto is now in Shikamaru's body.
I'm not sure as the title doesn't give a clear indication. It could be the reborn character who is into gaming, perhaps a mentor figure who helps them navigate the gaming world, or even rivals within the game.
Naruto himself is often a central and very popular character in these fanfictions. Since it's about his rebirth, readers are interested in seeing how his character evolves differently.
Sakura also remains popular. In 'fem naruto reborn fanfiction', she might have a different kind of friendship or rivalry with Fem Naruto. And Kakashi is still well - liked. His role as a mentor to Fem Naruto can be really interesting to explore, especially how he might adjust his teaching methods to suit a female Naruto.
In 'Reborn into Naruto World with Tenseigan Novel', the main character is the person reborn into the Naruto world with the Tenseigan. This character will have to navigate the complex world of Naruto, dealing with the various ninja clans, politics, and battles. There may be a mentor - like figure who helps the main character understand and control their newfound power. And then there will be antagonists, perhaps jealous of the main character's power or having their own evil plans for the Naruto world. The interactions between these characters are what drive the story forward.