Another important character could be Kakashi. He has a lot of knowledge and experience. He could guide Naruto on how to deal with the Juubi, especially with his knowledge of different jutsus and battle strategies. Maybe he gives Naruto some crucial advice that helps in the sealing.
Sasuke is likely to be an important character. His power and his relationship with Naruto could play a big part in the story. He might help Naruto in the fight against the Juubi or have some other role related to the sealing process.
The Nine - Tailed Fox, Kurama, is also bound to be important. Since it has a connection with Naruto and is also related to the concept of the Juubi. Kurama could provide Naruto with extra power or some insights on how to handle the Juubi. For example, it might share some ancient knowledge about the origin of the Juubi that helps Naruto in the sealing process.
Well, perhaps in the fanfic, Naruto has to team up with his friends in a unique way to seal the Juubi. For example, Sasuke could use his Sharingan to help Naruto locate the weak points of the Juubi while Naruto does the actual sealing. And Sakura could use her medical ninjutsu to keep Naruto in top shape during the process.
The idea of Naruto sealing the Juubi is quite common in Naruto fanfictions. If Kurotsuchi is involved, it could be that the author wants to explore a different kind of relationship. For example, Kurotsuchi could be from a different village and her interaction with Naruto during this momentous event could lead to a new alliance or at least a better understanding between their respective villages. Also, the sealing of the Juubi by Naruto is no easy feat. It would require a great deal of chakra control and knowledge of sealing jutsus. Kurotsuchi, with her own skills in earth - based techniques, might be able to contribute in unexpected ways, like creating a stable environment for the sealing to take place.
Iruka could also be important. Since he was one of the few people who showed some kindness to Naruto in the original story. He might feel guilty about Naruto leaving due to neglect and set out to find him. His presence could add an element of emotional depth to the story as he tries to make amends.
There could be a kunoichi who is a master of stealth and espionage. She might be the first to discover Naruto's arrival in the kunoichi world. Another main character could be a kunoichi with a strong sense of justice, similar to Naruto. She would be the one to advocate for Naruto when others are against him. And perhaps there's a kunoichi who has a special connection to an ancient power in this world, and she and Naruto's Juubi power somehow interact in an unexpected way throughout the story.
Obviously, Naruto is one of the main characters as the title implies he is the one sealing the Kyuubi. Also, the Kyuubi itself is a main character in a sense. It has a major role in the story as it is the entity being sealed.
I'm not sure since I haven't read it. It could be Naruto himself, as he often has to deal with different seals and his own fate. Or it could be a new original character created specifically for this fanfic who has a special connection to the seals of fate.
There could be other characters too, like the Sage of Six Paths. He might play a crucial role in the Juubi's birth in the fanfic. And also, villains from the Naruto universe could be involved, as their actions might have either directly or indirectly led to the birth of the Juubi.
Naruto himself is often involved in Naruto seals fanfic. Since he's the main character in the original series, writers like to explore his potential with seals further. Sasuke is also a popular choice. His Uchiha heritage and his own power - hungry nature make for interesting stories when combined with seal knowledge. Kakashi is another one. His Sharingan and his vast knowledge of jutsu could lead to some really creative seal - related scenarios. And of course, Jiraiya, as he was a master of many things including seals, and his relationship with Naruto can be further developed in fanfic with a focus on seals.
From the Selda side, Link could be a main character. He is used to fighting against great evils in his own world. If the Juubi were to invade, Link would be a natural ally for Naruto and the others. His skills with the sword and his ability to navigate through different terrains would be very valuable in a battle against the Juubi - influenced threats in the Selda world.
There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.