Definitely Naruto. And it could also feature Kakashi. Kakashi has been an important mentor figure to Naruto. In a story about bonds that can't be remade, perhaps there is a rift between them that can't be fixed. Another possible main character could be Sakura. She has her own complex relationships with Naruto and Sasuke, and this fanfiction might delve into how those relationships change when certain bonds can't be mended.
Naruto is the obvious one. Since the story seems to be about broken bonds, characters like Gaara could also be important. Gaara had a difficult past and his bond with Naruto was one of the first real connections he made outside of his own village. If there are bonds that can't be remade, it could be about Gaara's relationship with others in his village or even his newfound friends in Konoha. Additionally, Hinata might be involved as she has deep feelings for Naruto, and if there are unbreakable rifts, it would affect their relationship too.
I'm not sure specifically as I haven't read this exact fanfiction. But generally, in Naruto fanfictions, it could be about Naruto dealing with broken relationships or past traumas that prevent certain bonds from being mended. Maybe it involves his friends who have turned against him, or old enemies whose enmity can't be overcome.
There could also be new characters introduced. Maybe a business rival who is jealous of Naruto's success. This rival could be from another village or even an outsider who has his own motives for trying to take down Naruto's billion - dollar empire. And of course, the Hokage would still play a role, as Naruto's actions as a billionaire would have an impact on the village's politics and power structure.
Definitely Naruto himself. He'd be a central character, causing chaos and bringing energy to the daycare. And Sasuke would also be there, probably being a bit of an aloof kid that Naruto is determined to get close to.
Obviously, Naruto would be one of the main characters. Since it's a Naruto - centered fanfiction with the beyonder element added. Sasuke could also be a main character as he is often Naruto's counterpart and rival. They might both be involved in exploring the beyonder powers. Additionally, characters like Kakashi could play important roles. He could be the one guiding Naruto and Sasuke in understanding these new and strange beyonder powers, given his experience and wisdom.
Other Naruto characters like Sasuke or Sakura might also be important. Sasuke could be involved in Naruto's new role as a 'guardian' either as a supporter or a skeptic. Sakura, with her medical skills, could play a role in helping Naruto in his 'guardian' - related adventures, perhaps by healing those affected by new threats that come with the combination of the two universes.
Besides Naruto and Sasuke, Sakura could also be a main character in such fanfiction. Her medical ninjutsu skills might be needed to heal the wounded during the battles against Galactus. And there could be some original characters created specifically for the fanfiction who have some special knowledge or power related to Galactus and the Naruto world.
Obviously Naruto and Ophis. Naruto is the main protagonist from the Naruto series, and Ophis is from another fictional context. There could also be some of Naruto's friends like Sasuke or Sakura who might be involved in some way. Maybe they are worried about Naruto when he disappears into Ophis' world.
I think Kakashi would be an important character too. He could be Naruto's mentor in this new situation, teaching him how to use his remaining abilities effectively. Also, Sakura might be there, using her medical ninjutsu skills to try and help Naruto with his crippled state.
The main characters would be Naruto and Axel. But there could also be other characters from the Naruto universe like Kakashi. Kakashi might be suspicious of Naruto's new abilities and try to figure out if they are a threat or not. And from Axel's side, his friends like Roxas could also make an appearance. They might initially be hostile towards Naruto but then learn to accept him as he proves himself.
There could be new characters introduced in this fanfiction. Maybe a mysterious ninja from a far - off land who knows about Naruto's secret and either helps or hinders him. Another possibility is that an old, forgotten character from the Naruto universe is brought back and has some connection to the secret.