The teachers at Hogwarts, like Dumbledore (if he's still alive in the fanfic), would be solemn. Dumbledore might see it as a great loss to the wizarding world. Hagrid would be inconsolable. He had a special bond with Harry since his first year at Hogwarts. The other students would be in shock. Some might be inspired by Harry's memory to be braver, while others would be scared about what the future holds without him.
Hermione would likely be devastated. She might go into a deep state of mourning, but also use her intelligence to try and figure out what led to Harry's death. Ron would be angry and heartbroken at the same time. He'd probably want to seek revenge if there was someone or something responsible.
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.
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.
Uraraka would be heartbroken. She was one of Deku's closest friends. She might lose some of her usual cheerfulness and become more focused on finding out who or what was responsible for his death. She could also struggle with her own self - confidence in using her quirk, as Deku was always an inspiration to her.
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 wizarding world would be in chaos. They would realize too late that they had pinned all their hopes on Harry and neglected him. There would be a great sense of guilt and despair. The Hogwarts students would be shocked and many would feel a deep sense of loss. The teachers might question their own actions and inactions that led to this. The Order of the Phoenix would be in disarray, having lost their main weapon against Voldemort.
Ruby would be deeply saddened. She might blame herself for not being able to protect Blake. Weiss would probably be in shock at first, then become more determined to fight against whatever caused Blake's death. Yang would be furious and seek revenge. She has a very close bond with Blake, and her death would likely fuel Yang's anger towards the enemy.
Sakura would be devastated. She has always seen Naruto as a dear friend and teammate. She would probably blame herself for not being strong enough to prevent his death. Sasuke, on the other hand, might feel a deep sense of loss and anger. He might go on a solitary journey to find out who was responsible.
The characters would be devastated. Grover, Percy's best friend, would be heartbroken. He might lose his ability to find Pan because of the emotional turmoil. His search for Pan was always connected to his friendship with Percy, and without Percy, he could be lost.
Nagisa would be completely devastated. She has a very deep connection with Tomoya, and his death would likely cause her to sink into a deep depression. She might even lose the will to live, especially if it happened suddenly. For example, if he died in an accident, she would be left with a sense of emptiness and longing for him.
The other members of Team RWBY would be deeply affected. They would start to question their own abilities and the purpose of their fight. Some might withdraw into themselves, like a broken Weiss who shuts out the world for a while. Yang, on the other hand, could become reckless in her pursuit of vengeance, endangering herself and others. Blake would try to hold the team together, but she would also be mourning privately. The other friends and allies in the RWBY universe would also be in mourning. Qrow might blame himself for not being there to protect Ruby, and Ozpin would see it as a great setback in the fight against evil.