If Harry dies in the final battle in fanfiction, the Order of the Phoenix members would be devastated. They had placed so much hope in Harry. Sirius, if still alive in the fanfiction, would be enraged. He would see Harry as a son - like figure and would be hell - bent on making Voldemort pay. Lupin would be solemn and would likely try to keep the remaining members of the Order focused on the task at hand. And Hagrid? He would be inconsolable. Harry was like family to him, and he would probably feel lost without him, but also be ready to fight to protect Harry's memory.
Ron would be completely shattered. Harry was his best friend, and he would likely feel guilty for not being able to protect him. He might become reckless in his attempts to avenge Harry. Hermione would be grief - stricken but also determined. She would pour herself into finding a solution to the mess left behind. Ginny would be heartbroken. She might initially withdraw but then become a strong advocate for justice, using her own skills and magic to fight against Voldemort's regime.
The other students at Hogwarts would be in shock. Harry was their hero. Younger students would be scared, while the older ones might feel a sense of duty to carry on Harry's fight. Dumbledore's Army members would be especially affected. They would be angry and sad at the same time. Neville, for example, might feel like he has let Harry down, but also be more resolved than ever to oppose Voldemort. The teachers at Hogwarts, like McGonagall, would be deeply saddened but also know that they have to keep the school together and protect the students from the chaos that would follow Harry's death.
Dumbledore would feel a great sense of loss and guilt. He had always seen Harry as the key to defeating Voldemort. His reaction could be to go into seclusion for a while to think about what went wrong. The other Hogwarts professors would be in shock. Hagrid would be heartbroken and might even leave Hogwarts in his grief. And the Death Eaters would be jubilant, but also a bit wary as they know that there could be unforeseen consequences to Harry's death.
If Hermione dies in the final battle in fanfiction, the story would change drastically. For one, Harry and Ron would lose their intellectual backbone. Hermione has always been the one with the plans and knowledge, so without her, they'd be floundering. The whole wizarding world would also feel the loss deeply as she was a key figure in the fight against evil. Her death would be a huge blow to the morale of the Order of the Phoenix and all those fighting against Voldemort.
For Harry, it would be devastating. Hermione was like a sister to him. He'd likely become more isolated and perhaps more determined than ever to defeat Voldemort, but also more reckless. Ron would be heartbroken. He had a special relationship with Hermione, and her death might make him feel like he failed her. Ginny would also be affected. She looked up to Hermione, and her death would be a shock to Ginny's sense of security in the group.
Ron and Hermione would be completely shattered. Ron, who had been Harry's friend since the beginning, would probably be in denial at first. Hermione would likely try to figure out what exactly happened. Dumbledore would be solemn and would try to keep the school calm while also investigating the matter. Hagrid would be inconsolable, crying his eyes out. The other students would be in a state of shock and sadness. The teachers would also be deeply affected, especially those who had taught Harry or had some connection with him.
One possible plot development could be that with Harry's death, Voldemort fully takes over the wizarding world. His reign of terror becomes absolute, and the remaining characters have to go into hiding or form a secret resistance. For example, Hermione might lead the resistance, using her intelligence to try and find a way to defeat Voldemort without Harry. Another aspect could be the emotional turmoil among the characters. Ron would be devastated, and his character could either sink into depression or be driven by a thirst for revenge. Ginny, too, would have to deal with the loss of her love and might become a more hardened and fierce fighter against the Dark forces.
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.
Rias would be devastated. Issei was not only her servant but also someone she had deep feelings for. She would likely go into a rage at first, vowing to find and punish whoever was responsible. Akeno would be heartbroken as well. She and Issei had a special bond, and she would be in mourning for a long time. Kiba might feel a sense of guilt, wondering if he could have done more to protect Issei. Asia would be in shock and constantly pray for Issei's soul, hoping for some kind of miracle.
Harry would be crushed. He'd feel like he lost a sister. Ron would be in total despair, probably blaming himself. Ginny would be heartbroken too as Hermione was like a sister to her. The Weasley family, who had accepted Hermione as one of their own, would be in mourning. They'd all wonder how they could have prevented it and would be angry at the unfairness of it all.
He might be shocked. The fanfiction could show his friends or himself in situations that are completely different from what he knows. For instance, if it shows him losing to a minor villain, it would be very unexpected for him.
Bakugo would be deeply affected. He might initially deny his feelings, but then would be filled with guilt for all the times he was mean to Deku. He could become more withdrawn and focused on becoming a hero worthy of Deku's sacrifice.