The main character is Naruto, of course. But in addition to that, there could be new characters created specifically for this fanfic. For instance, a spirit guide who helps Naruto understand and control his ability to see the dead. And some of the more minor characters from the original series might also become more prominent if they have some connection to the dead or to Naruto's new situation, like Ino who might be able to sense something different about Naruto.
Obviously, Naruto would be one of the main characters since it's about him seeing dead people. There might also be other characters like Sasuke, who could react to Naruto's new ability in different ways. And perhaps some of the dead characters he sees, like the Fourth Hokage, could also be considered main characters in the context of this fanfic as they play important roles in Naruto's story.
In the 'naruto sees dead people fanfic', it might be that Naruto's encounters with the dead are used to reveal hidden aspects of the Naruto universe. For example, he could meet dead villains and understand their motives better, or he could see the spirits of his ancestors who guide him in his journey to become a stronger ninja. It could also be a way to bring back fan - favorite characters in a different way, even if they are dead in the original series.
Naruto is the central character, no doubt. Hinata could be important as well. If Naruto sees a future related to his family or his relationship with Hinata, she will play a significant role. Additionally, the Hokage, like Tsunade or whoever is in charge at that time, would be important as Naruto may need to report the future he saw to the village leadership to take collective action.
Most likely Naruto himself would be one of the main characters. Since it's a Naruto - based fanfic, he would bring his ninja skills and determination to the high - school - zombie scenario.
It could be a dark ninja who also has the ability to manipulate spirits. He might use this power to create chaos in the ninja world and Naruto has to stop him while also dealing with the spirits he sees.
Kakashi could be another important character. As a mentor figure, he might be guiding the younger generation through the difficult times after Naruto's death, using his experience and wisdom to keep the village stable.
Knuckles might also be part of the story. As the guardian of the Master Emerald, he has a sense of responsibility. He could either be involved in the events that led to Sonic's fall or be part of the rescue mission. His strength and knowledge of the area (if Sonic falls in a location related to the Master Emerald) could play an important role.
Obviously, Naruto would be the main character since it's about his past seeing the future. Sasuke would also be important as Naruto's relationship with him is central to the Naruto series. Kakashi too, as he is Naruto's teacher and mentor, and his guidance might be influenced by Naruto's new - found knowledge of the future.
It could be a new character, perhaps someone from a far - off land who has a grudge against Naruto's future self for some reason. This character might be extremely powerful and has plans to destroy everything Naruto holds dear. When Naruto sees this future, he has to find a way to stop this new threat before it even begins to form.
Naruto is surely a main character. Additionally, there might be some original characters created specifically for this fanfiction. For example, there could be a powerful spirit or a guide in the world of the dead who helps Naruto understand his new powers. And characters like Kakashi could also be involved, perhaps providing advice or assistance to Naruto as he deals with the implications of being the king of the dead.
It's hard to say for sure as it's fanfiction, but it could feature Naruto himself. He might be on a journey to understand the secrets of the book of the dead. Sasuke could also be involved as he often gets involved in Naruto's important quests. Maybe even Kakashi could be there, guiding the younger ones with his wisdom about the strange new elements introduced by the book of the dead.