It could be Sasuke. If Sasuke decides to fully embrace the dark side and wants to bring chaos, he might target Naruto specifically. His actions and taunts could be what drives Naruto insane.
Obviously Naruto himself is the main character as it's about him being driven insane. But often, Sasuke can also be involved as his actions or relationship with Naruto can be a trigger for Naruto's insanity.
The main antagonist could be Naruto himself in a sense. Since he has gone insane, his actions are now unpredictable and harmful to the people around him. His former friends, like Sakura or Kakashi, might have to step up and try to stop him, seeing him as the new threat.
It could be that Naruto discovers some dark secrets about his past, such as the real reason his parents died or some hidden and malicious intentions of the village elders. This overwhelming knowledge could be too much for him to bear and push him over the edge into insanity. He would then start to act erratically, causing turmoil in the ninja world.
A plotline could be that Naruto is exposed to a powerful genjutsu that makes him lose his grip on reality. As he tries to fight it, he gradually goes insane. He could start seeing hallucinations of his worst nightmares, like the death of all his friends. This could cause him to lash out at those around him, thinking they are part of the genjutsu, and his actions would be very different from his normal self.
It could be an unknown force that manipulates the events around Naruto to drive him insane. This force could be an ancient evil that has a vendetta against the Uzumaki clan. It secretly plants seeds of doubt and chaos in Naruto's life, causing him to question everything he knows and trusts, slowly pushing him over the edge.
It could be a new dark wizard who has the power to manipulate minds. This wizard might target Harry specifically, driving him insane as part of a larger plan to take over the wizarding world. Since Harry is seen as a hero, if he goes insane, it would create chaos and make it easier for the antagonist to achieve his goals.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
Perhaps in the fanfiction, some sort of dark magic or curse has been placed on Naruto. This could be messing with his mind and causing him to act in an insane manner. It could be a way for the author to introduce a new and dark plot element to the story. Another cause might be that he has been betrayed by those he trusts the most, which shatters his mental state.
Madara Uchiha could also be a likely cause. If he somehow managed to return or had a long - planned scheme that involved breaking Naruto's sanity. He was a very powerful and devious foe who could use his knowledge of the ninja world and its powers to target Naruto's mental stability.
Obviously, Naruto and Hinata are the main characters. Naruto, the hero with a big heart but perhaps in this fanfiction, he has some insane tendencies. Hinata, on the other hand, is his love interest. She's the one who will likely be there to support him, understand him, and maybe even save him from his 'insane' self. There could also be other supporting characters from the Naruto universe like their friends or mentors who might play a role in how their relationship unfolds in this fan - created story.