Other characters could be involved in various ways. For example, Sasuke might try to interfere with the Dead Demon Consuming Seal when Naruto is about to use it. He could either try to stop Naruto out of concern for his friend's life, or he might be manipulated by an enemy to disrupt the seal, thinking it would give him an advantage. Another way is that a character like Kakashi, with his knowledge of seals, could try to modify the Dead Demon Consuming Seal's power to help Naruto. He might find a way to make the seal more effective or less dangerous to Naruto.
A possible plot twist is that the Dead Demon Consuming Seal actually has a sentient side to it. When Naruto uses it, instead of simply sealing away the enemy, the sentient part of the seal decides to make a deal with Naruto. It offers to give him unlimited power in exchange for his body becoming a vessel for all the demons in the seal. Naruto has to decide whether to accept this power at the cost of his humanity or find another way to defeat his enemy without relying on this dangerous deal. This would create a huge moral dilemma for Naruto and also change the dynamic of his relationships with his friends and the village.
The curse seal often gives characters increased power. For example, they might become stronger physically, able to take on tougher opponents. In some fanfiction, it can also change their appearance, like making their eyes glow or their skin take on a different hue.
Obviously Naruto himself is a main character as the memory seal is related to him. Sasuke could also be involved, as he has a deep connection with Naruto and might notice the changes in him due to the seal.
In many fanfictions, Naruto might use his unique chakra, perhaps enhanced by his Uzumaki heritage or the Nine - Tails' power. His strong will and connection to his friends could also play a role, allowing him to find a way to break the seal through sheer determination and his belief in Neji's potential.
There could be a situation where an ancient technique is awakened within Hinata. This technique might have been long forgotten, but due to certain events in the fanfiction world, it resurfaces. This ancient technique could be the key to her being able to control Naruto's seal. It might involve her chakra nature or some spiritual connection she has with the seal's origin or the entity that placed the seal on Naruto in the first place.
Naruto's life would be very different. He would be more respected in the ninja world for his unique skills. Instead of just being known for his Nine - Tails chakra, his sealing abilities would also be a major factor. He could use his sealing skills to protect the village in new ways, like creating powerful seals around Konoha to prevent enemy attacks. Also, he might be called upon more often to deal with dangerous artifacts or threats that require sealing.
One possible consequence could be a change in Naruto's power level. If Hinata can control the seal, she might be able to regulate the flow of power from it, which could either enhance or limit Naruto's abilities.
Another aspect in such fanfictions could be the origin of the Kyuubi seal. Some writers might delve into ancient ninja secrets and forgotten jutsu that were used to create the seal in the first place. They might also explore if there were other possible ways to seal the Kyuubi that were considered but not used. This can add a whole new layer of mystery and intrigue to the Naruto universe.
Perhaps there is a wise old seal master who takes the prodigy under their wing. This character could have a lot of knowledge about the history and secrets of seals, and they would play an important role in the prodigy's growth. Additionally, there could be some antagonists, maybe a group of rogue ninjas who want to steal the prodigy's seal knowledge for their own evil plans.
Most likely Naruto would be one of the main characters since it's in the title. And since it's about a faulty seal, characters who are related to Naruto's situation in the original story might also be main characters, like Jiraiya. He was Naruto's teacher and knew a lot about the seals, so he would probably play an important role in this fanfiction.