Sasuke is likely to be involved. Given their complex relationship, Sasuke might be on the receiving end of Naruto's rage at times or be someone who tries to understand and calm him. Kakashi could also be there, as Naruto's sensei, he would surely be involved in dealing with Naruto's rage.
Iruka could be involved too. Since he was one of the first to really connect with Naruto in the original story. He might be the one to reach out to Naruto when his rage gets out of control. And Sakura, as part of Naruto's team, would be affected by Naruto's rage and might try to help him in her own way.
Sasuke is often involved. Since he and Naruto have a complex relationship, his reaction to Naruto's demon form can be a major plot point. He might be initially suspicious or even see it as a new way to compete with Naruto. Sakura could also be there. She might be worried about Naruto and try to find a way to help him control this new form.
Sasuke could be involved. Since they have a complex relationship, Sasuke might either be a rival during Naruto's training, pushing him to be better, or he could end up joining Naruto in some training sessions to become stronger together.
Sasuke is an obvious choice. He has a connection to Naruto that goes beyond just friendship or rivalry. He might be the one who first discovers Naruto's predicament and sets out to find a solution. Sakura is essential too. Her skills as a medical ninja are crucial. She could be the one who patches Naruto up every time he gets injured in his fight against whatever is threatening his life. Also, characters like Jiraiya might make an appearance. Even though he's dead in the original story, in this fanfiction, he could come back as a spirit guide to help Naruto find a way to 'refuse to die', giving him advice based on his own knowledge and experiences.
Sasuke is likely to be involved. He and Naruto have a complex relationship and Sasuke has become an ally. He could offer his strength in combat to protect the rebuilding efforts. Also, Sakura. She's a skilled ninja with medical abilities that could be useful if there are any injuries during the process.
There might be characters from Iwa who oppose Naruto. For example, the Tsuchikage of Iwa would surely be involved as the leader of the village Naruto is attacking.
Gaara could be one of the characters. He has a bond with Naruto and if Naruto is against Konoha, Gaara might get involved to help Naruto or at least try to mediate. Shikamaru is another likely character. His intelligence would make him a key figure in trying to understand Naruto's motives and also in devising strategies for Konoha. Hinata might be involved as well, because of her feelings for Naruto. She might try to protect Naruto from Konoha's attacks.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
Some fanfictions might also include characters like Kakashi in important roles. As Naruto's former teacher, he could be a guiding figure or someone who has to adapt to Naruto's new level of power. He might also be involved in trying to keep Naruto's power in check if it starts to have negative effects on the ninja world.
Naruto himself is often a main character in naruto helmet fanfiction. Since he is the protagonist of the original series, it makes sense that he would be involved with a special helmet in fan - created stories. He could discover the helmet's powers and use them to become even stronger.