Maybe it's his father, Minato. Minato has a lot of things to tell Naruto, like more about the Uzumaki clan and how to better use the power of the Nine - Tails. And as his father, their connection is very strong, so it's quite possible that he is among the first dead Naruto talks to.
Perhaps it's the Third Hokage. The Third Hokage knew a great deal about the village and its secrets. He could start guiding Naruto on how to protect Konoha in a different way now that Naruto can talk to the dead. Also, he might give Naruto some insights into handling political matters within the village.
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.
Well, it could change the plot in many ways. For example, he might get some crucial information from the dead. Maybe he could learn some long - lost jutsu from a dead ninja master, which would make him stronger and help him in battles. Also, he could get some secrets about the Akatsuki's plans from their deceased members, which would give Konoha an advantage.
It varies from one fanfiction to another. Some fanfictions might depict Naruto as dead to introduce a new set of challenges for the remaining characters. For example, if Naruto dies, Konoha might be left vulnerable, and characters like Sasuke or Sakura would have to step up in different ways. But there are also plenty of fanfictions where Naruto remains alive and continues his journey, perhaps facing new and even more dangerous threats than in the original series.
Iruka Sensei could also be a candidate. He was one of Naruto's first supporters and has a deep connection with him. He would probably risk a lot to save Naruto.
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.
One of the most interesting could be the ghost of the First Hokage. He has so much knowledge about the founding of the village and the early days of the ninja world. Naruto could learn a great deal from him, like how to better lead and protect the village.
Kakashi could also play a role. He has always been a mentor to Naruto and his sense of duty and loyalty to his student might drive him to search for Naruto when he hears that he's been left for dead. He has the skills and knowledge to find and save Naruto in a difficult situation.
Obviously Naruto would be one of the main characters. Depending on the plot, Sasuke could also be involved, either as a helper or an obstacle. Sakura might also play a role, perhaps as a healer trying to save Naruto. There could also be new characters created specifically for this fanfiction, like a dark figure who is responsible for Naruto's 'dead man walking' situation.
Obviously Naruto would be one of the main characters as it's in the title that he's 'broken'. And since Akatsuki is dead, they might still have an impact on the story through flashbacks or the consequences of their actions. Other characters like Sasuke could also be main characters as he's often closely related to Naruto's story.