Hermione would be extremely shocked and disappointed. She has always been a firm believer in the fight against Voldemort and would see Harry's alliance as a huge betrayal. She would probably try to talk Harry out of it at first and if that fails, she might start her own investigation to find out what made Harry do this.
Ron would be angry. He might feel that Harry has gone mad. Ron has grown up in a family that has always been against Voldemort. He would not understand Harry's decision at all and could even become hostile towards Harry for a while. He might also try to convince the rest of the Weasley family to help him bring Harry back to the right side.
The Order of the Phoenix would be in chaos. Dumbledore, if still alive, would be deeply troubled. He would try to figure out the reasons behind Harry's choice. Some members might want to capture Harry and break his alliance with Voldemort by force, while others might think there is a deeper mystery that needs to be unraveled first.
Voldemort would be calculating. He might be using the Armistice as a way to regain his strength or plot a new strategy. His Death Eaters would be divided. Some might be happy to have a break from the fighting, while others would be eager to continue the war. Dumbledore's followers would be watching closely too. They would be the ones to keep the balance and make sure that the Armistice doesn't turn into a way for Voldemort to take over completely. The Ministry of Magic would be in a difficult position, trying to enforce the Armistice while also dealing with the different factions within the wizarding world.
Ron could be another important ally. His loyalty and courage would be crucial. He might be the one to act as a decoy sometimes, drawing attention away from Harry when he is on a dangerous spying mission. Also, Ron's ability to blend in with the wizarding crowd could help Harry gather information without being too conspicuous.
Well, Ginny would be heartbroken that Harry left. She might have expected to build a life with him after Voldemort was defeated. But she's also a strong character, so she would likely throw herself into her own life, perhaps becoming a star Quidditch player. Neville would be overjoyed that Voldemort was dead. He might take on a role at Hogwarts, teaching herbology perhaps, and sharing his knowledge with the students. As for the other characters in the wizarding world, those who had suffered under Voldemort would be in a state of jubilation and start to pick up the pieces of their lives.
The other students at Hogwarts would be very afraid. Some might avoid Harry out of fear that Voldemort could use him to harm them. But Neville, being brave, would probably try to reach out to Harry in some way, believing that there's still good in him. The Order of the Phoenix would be in chaos at first. But then they'd start formulating plans. Lupin would be worried but also think rationally about how to deal with the situation, using his experience as a werewolf and a wizard.
The teachers at Hogwarts would have diverse reactions. McGonagall might be disappointed but would still hope that Harry could resist the dark influence. Snape would be extremely complex in his reaction. Given his history with both Harry and Voldemort, he might see it as a sign of the prophecy coming full circle. The Death Eaters, on the other hand, would see Harry as a potential new leader. They might try to persuade him to embrace the dark side. As for Harry's other friends like Neville, he would be worried but would also stand by Harry, believing that he could make the right choice. The reactions of these characters would create a lot of drama and tension in the fanfic.
For the Order of the Phoenix members, they would be in a state of shock. Lupin, for example, would be very concerned about Harry. He would try to talk to Harry, offer his support, but also be on guard. Ginny would be heartbroken. She loves Harry, but the thought of him being Voldemort's heir is terrifying. She would try to hold onto their relationship while also being afraid of what Harry might become. And the Death Eaters? They would be excited at first, thinking they have a new leader, but as they get to know Harry better, they might realize he's not as easy to manipulate as they thought.
The wizarding world would be in shock. Harry is seen as their last hope, so his surrender would make many lose faith. The Order of the Phoenix might go into a state of chaos, trying to figure out how to rescue Harry and continue the fight against Voldemort without him.
Harry would probably be in a state of shock and disbelief at first. He'd be filled with a mix of fear, confusion and anger. Voldemort might be initially dismissive or see it as an opportunity to further his own dark plans. Hermione would be practical, trying to find solutions in books and magic. Ron would be more likely to be openly disgusted at first but might come around to support Harry as a friend.
Snape could also be a character in such a fanfic. His complex relationship with both Harry and Voldemort makes him an interesting addition. He might be in a difficult position, having to keep his cover as a double - agent while also trying to find a way to save Harry without revealing his true intentions to Voldemort.
There would likely be a great deal of anger and a thirst for revenge. Harry has many friends and supporters. They would probably form some sort of resistance against Voldemort, but they would also be mourning Harry's death deeply. Some might even question if there was more they could have done to prevent it. The Ministry of Magic would also be in a state of turmoil, as they had been relying on Harry to end the threat of Voldemort.