The other Hogwarts students would be a mix of scared and curious. Some might avoid Harry altogether, fearing that they could be in danger. Others, who are more brave or naive, might try to approach him to see if they can help. The teachers would be in a difficult position. They would need to protect the other students but also try to find a solution for Harry. For instance, Professor McGonagall might be torn between her loyalty to Harry and her duty to keep the school safe.
Often in fanfiction, their relationship can change drastically. They could end up as allies, which would require both of them to change. Harry would have to see past Voldemort's evil deeds, and Voldemort would have to let go of his hatred for Harry. This exploration of character change makes fanfiction so interesting.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
Ron would be extremely angry at first. He might feel betrayed by Harry. But as he realizes that something must be seriously wrong with Harry, he would probably join Hermione in trying to save Harry from Voldemort's influence. They would look for a way to break whatever spell or false belief has hold of Harry.
Harry would likely be defiant at first. He has a strong sense of justice and would not easily submit to Voldemort's punishments. For example, if Voldemort uses the Cruciatus Curse, Harry would probably try to resist the pain and not show weakness.
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.
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.
Harry often becomes more conflicted. He has always seen Voldemort as the ultimate evil, but now being mates, he has to re - evaluate his views. He might start to see the humanity in Voldemort, which makes him question his own beliefs and actions in the fight against the Dark Lord. For example, he may hesitate in battles more than before.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Hermione would probably be very supportive. She's always been open - minded and a strong advocate for equality. She would likely help Harry with any practical aspects of the transition, like finding appropriate clothes or dealing with the different expectations in classes. Ron, on the other hand, might be a bit more confused at first. He's used to Harry as a boy, but eventually he would come around and still be a loyal friend.
Harry Potter is central as the possessed. Voldemort is crucial as the possessor. Harry's friends are bound to be main characters. Hermione, with her intelligence, would be constantly observing Harry's strange behavior and coming up with theories. Ron, being Harry's loyal friend, would be by his side and ready to fight for him. Also, characters like Sirius Black, if he's still alive in the fanfiction's timeline, could be involved. He has a close connection to Harry and would surely want to help him get rid of Voldemort's control.