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.
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.
Ruby would be devastated. Qrow is her uncle and has been an important figure in her life. She might initially go into denial and then be filled with rage, vowing to avenge his death. She could become more determined than ever to take down Salem, seeing it as the only way to make his sacrifice meaningful.
Optimus Prime might be deeply affected. He would probably feel a great sense of loss, as Arcee was one of his loyal soldiers. He might give a solemn speech about her bravery and sacrifice, while also mourning privately.
The Weasley family would also be greatly affected. Mrs. Weasley would surely mourn Harry as one of her own. Ginny, who had feelings for Harry, would be heartbroken. She might become more fierce in her pursuit of justice in the wizarding world. And the other students at Hogwarts, like Neville, would be inspired by Harry's sacrifice to step up and be braver in the face of danger.
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.
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.
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.
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.
Winry would be heartbroken. She loves Edward deeply. She might initially go into denial and then be filled with anger at the circumstances that led to his death. She could also turn her grief into a determination to help others with her mechanical skills, in memory of Edward.
Sasuke would be deeply affected. He and Naruto had a complex relationship. He might feel a sense of regret for all the times he had pushed Naruto away. His reaction could range from anger at the situation to a new - found respect for Naruto's sacrifice.