Well, it might start with Naruto distancing himself from his friends. He stops going on missions with them, and he doesn't show up at their usual gathering places. For instance, he might avoid Ichiraku Ramen, which was always a place of connection for him and his friends.
When Naruto closes his heart, the fanfic could show his internal struggle. He might have flashbacks to all the good times with his friends and then feel guilty for pushing them away. But at the same time, he feels like he has no other choice. His daily routine would change completely. Instead of his usual energetic self, he becomes a loner. He spends his time alone in places like the forest on the outskirts of the village, brooding. His appearance might even change, with a more sullen look on his face. And as his friends try to get through to him, they might discover new things about Naruto's past or his true feelings that were hidden before, which could be a turning point in the story.
As the story progresses, his friends would likely start to worry and try to reach out to him. But Naruto, with his closed heart, would push them away. This could lead to some conflict among his friends as they argue about the best way to help him. Some might think they should give him space, while others want to force him to open up. In the meantime, Naruto's skills might start to deteriorate because his emotional state affects his ability to use his chakra effectively. And this could put the village in danger if there are threats around. Eventually, through some big event, like a major attack on the village where Naruto realizes he needs his friends and the power of their bonds, he may start to open his heart again.
Perhaps after facing too much betrayal from the village he loves. If the villagers turned on him despite all his efforts to protect them, it might lead him to close his heart in self - defense, not wanting to be hurt again. He's always tried so hard to gain their acceptance and if that was all for naught, it would be a huge blow.
Self - reflection is key. If Naruto takes the time to really look at himself and understand why he closed his heart in the first place, he can start to work through those issues. Maybe he realizes that closing his heart is only causing him more pain in the long run and that by being open, he can experience the full range of human emotions again, both the good and the bad.
It could also be due to the constant pressure and expectations on him. He's always had to be the hero, the one to save the village. If he feels he's constantly failing or not living up to those expectations, he may close his heart to protect himself from that self - doubt and disappointment. In the fanfic, perhaps there are new threats to the village that he feels he can't handle, and this leads to him shutting down emotionally.
The story could take a dark turn. Without his heart, Naruto might become a soulless being, and his friends would be devastated. They might try to find a way to get his heart back. Maybe they search for a powerful sage or some ancient magic that could reverse what has been done.
A 'demon in his heart' in a Naruto fanfic could refer to many things. It might be about a character who has an inner struggle with a 'demon' - perhaps a dark side of their nature, like Naruto's struggle with the Nine - Tails. It could also be about a new character introduced in the fanfic who has a 'demon' within them that affects their actions and relationships with other characters in the Naruto universe.
One possible theme could be redemption. If a character has a 'demon in his heart', they might strive to overcome it and find redemption. Another theme could be self - discovery. As the character battles their inner 'demon', they may learn more about themselves.
Naruto might throw himself into his ninja training. He's always been dedicated to getting stronger, and heartbreak could fuel that drive even more. For example, he could spend countless hours perfecting new jutsu or sparring with his friends. Another way he could cope is by relying on his friends. Sakura and Kakashi, for instance, could be there to support him, listen to him vent, and offer advice. He might also find solace in eating ramen at Ichiraku. The familiar taste and the warm atmosphere of the ramen shop could give him some comfort.
In terms of his romantic relationships, if a love interest discovers the seal on his heart, it could either draw them closer or create distance. If they are understanding and want to help, it could strengthen their bond. But if they are afraid of the unknown associated with the seal, they might pull away. For example, Hinata might be initially scared but then resolve to stand by Naruto and help him understand the seal.
In the fanfic, Naruto's character could also become more introverted as he copes with the loss of his arm. He might need time alone to come to terms with his new reality. However, this solitude could also give him time to reflect on his life and his ninja path. He may emerge from this period with a new sense of purpose, perhaps focusing more on non - combat aspects of being a ninja, like diplomacy or teaching. This would be a significant shift from his previous more action - oriented self.
The story might develop with Naruto and Hinata facing some initial resistance from some conservative elements in the village who might have different ideas about mateship. But they would overcome it with their combined strength and love. Their relationship would become more intimate, and they would start planning for their future together.