For Hinata, it would be a devastating blow. She loved Naruto deeply. She might become withdrawn and lose her usual gentle strength. Shikamaru, being Naruto's friend, would be angry at first, feeling that Naruto's sacrifice was unnecessary. But then he would realize the gravity of the situation Naruto faced. His death would also impact the younger generation. Boruto, in this fanfiction's context, would grow up without his father, and this would shape his character in a different way, perhaps making him more determined to carry on Naruto's legacy.
One possible storyline could be that Naruto is faced with an overwhelmingly powerful enemy that he realizes can't be defeated any other way. So, in a selfless act, he decides to use the Reaper Death Seal. This would be a very tragic end for Naruto as he has so many friends and a family to live for. His death would likely send shockwaves through the Hidden Leaf Village and among his friends like Sasuke, Sakura, and Kakashi. They would be devastated, and it might also lead to a power vacuum in the village, causing all sorts of political and social unrest.
Perhaps an unknown hermit living in some far - off place. In many fanfics, there are mysterious characters that pop up with unique knowledge. This hermit could have mastered the Reaper Death Seal long ago and sees Naruto's potential, so decides to teach him.
For Hinata, it would be a crushing blow. She loved Naruto so much. She might withdraw into herself at first, overcome with grief. But then, she could find the strength within herself to carry on Naruto's legacy, especially for their son, Boruto (if in a post - Naruto - has - a - family scenario in the fanfiction). Shikamaru, being Naruto's friend, would feel a deep sense of loss. He would use his intelligence to try to figure out what led to Naruto's death and if there was any way to prevent it in hindsight. And the other members of the Leaf Village would be in a state of shock and mourning. Naruto was their hero, and his death would make them question their safety and future without him.
Tsunade herself would be in a state of shock and deep remorse. She might blame herself for Naruto's death, thinking that she should have been stronger and not needed saving. The villagers would also be greatly affected. The children who looked up to Naruto as a hero would be heartbroken. They would start to tell stories about his bravery, and some might even be inspired to become like him, despite his tragic end.
Sasuke would be deeply affected. He and Naruto had a complex relationship. He might feel regret for not being there to help Naruto and also feel a new sense of purpose in avenging his friend. Sakura would be heartbroken. She had always cared for Naruto, and his death would make her question the purpose of the ninja life. Haku herself would be in a state of shock and disbelief. She would feel a heavy burden of guilt, knowing that Naruto sacrificed his life for her. She might become more withdrawn initially but then use Naruto's memory as a motivation to become stronger.
The characters in 'Naruto' would be deeply affected if Naruto dies at birth in fanfiction. Jiraiya would be one of the most affected. He saw Naruto as his protege and the hope for the future. Without Naruto, Jiraiya might lose his enthusiasm for training and spreading his teachings. Tsunade would also be saddened as she had a connection to Naruto through her relationship with Jiraiya and Minato. The Rookie 9, Naruto's friends, would have a completely different dynamic. They might not form as tight a group without Naruto's energetic and positive influence. And the Sannin, as a whole, would have their relationships and legacies altered without Naruto in the picture.
Naruto might also try to remember Sakura in a positive way. He would cherish the memories they had together. He could start initiatives in the village in her name, like promoting medical ninja training more, since Sakura was a great medical ninja. He would slowly turn his grief into a force for good in the village.
Power and sacrifice are common themes. The death seal is a very powerful jutsu that often requires sacrifice, so many fanfictions explore this aspect. For example, a character might have to sacrifice their own life or something precious to use the seal effectively.
He would be filled with rage. Naruto has a strong sense of justice and the fact that someone took Hinata away from him would fuel his anger. He would vow to find and punish whoever was responsible. He might even become reckless in his pursuit of vengeance, endangering himself and those around him. His friends would have to step in to try and bring him back to his senses.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.