Naruto would probably be quite affected. He is someone who has been through a lot of hardships in his own life, so a sad fanfiction about himself might bring back some of those difficult feelings. He could be sad for a while, but then he would likely try to shake it off and remember that it's just a story.
Harry's reaction would depend on the nature of the fanfiction. If it's a cute, romantic fanfiction about Ron and Hermione, he might smile to himself and think it's kind of sweet. However, if it's something that distorts their characters or has a negative slant, he could get defensive. He might even try to find out who wrote it and why they would write such a thing about his friends.
He might be really confused. Fanfiction often contains wild and unexpected ideas that are different from his real life. He could be amused by some of the creative and out - there stories he overhears.
Naruto would probably have a range of reactions. He could be excited to see new adventures that people have written for him. But if the fanfiction was inaccurate about his character, like making him cowardly when he's known for his bravery, he would likely be a bit miffed. He might also be curious about the new relationships with other characters that the fanfiction presents.
Hinata would be completely devastated. She has always been deeply in love with Naruto, and his sacrifice would leave her heartbroken. She would still be strong for Boruto, though, and try to support him in any way she could.
The characters' reactions would be complex. For instance, Iruka Sensei, who was like a father figure to Naruto, would be inconsolable. He has watched Naruto grow from a troublemaker to a hero, and this sacrifice would be too much to bear. Shikamaru, being Naruto's friend, would feel a mix of admiration and sadness. Admiration for Naruto's selfless act but sad that he's gone. The kids in the village who looked up to Naruto would be inspired in a way. They would want to be like Naruto and protect the things they love, even if it means sacrificing themselves.
She would also be deeply emotional. Sakura has deep feelings for Naruto, even if they were not always fully realized or expressed. His sacrifice would make her confront these feelings head - on. She could be seen crying often, visiting Naruto's memorial frequently, and having long internal monologues about their relationship and what his sacrifice means to her.
He might be a mix of flattered and bewildered. Flattered because people care enough to write about him, but bewildered by some of the wild ideas in the fanfiction.
He might be surprised. He's used to his own life and adventures, and seeing it twisted and turned in fanfiction could be unexpected.
Well, Shikamaru would be one of the characters who would take Naruto's sacrifice very hard. He had always respected Naruto's determination. His reaction might be to become more serious about his duties in the village, feeling that he needs to step up in Naruto's absence. Hinata would be heartbroken too. She had deep feelings for Naruto. She might struggle to come to terms with his loss but also find strength in his selfless act. And all of the Konoha 11 would be united in their grief, vowing to protect the village and each other more fiercely in Naruto's memory.
His friends like Shikamaru would be deeply affected. Shikamaru, who had always respected Naruto's tenacity, would be at a loss. He might think about all the times Naruto pushed forward despite the odds and feel guilty for not being able to prevent his death. Hinata would be inconsolable. She had deep feelings for Naruto, and his sacrifice would shatter her world. The other ninja from different villages that Naruto had befriended during his journey, like Gaara, would also be in mourning. They would see Naruto's sacrifice as a noble act that inspired them to be better protectors of their own villages and to work towards a more unified ninja world.