Often, Harry is a complex character. So, his reaction could be a mix of emotions. He could be sad, angry, and confused all at once. He might try to find evidence to prove his friends wrong or to clear his name. He may also have an internal struggle, thinking about all the good times they had together and wondering how it all went wrong. This internal conflict could lead to him making some difficult decisions, like whether to try and salvage the friendship or cut ties completely.
He might be extremely hurt. Harry could go into a state of shock and disbelief at first. He may isolate himself, not wanting to trust anyone else for a while.
Harry could also get angry. He might lash out at his friends, demanding an explanation. He could use his magic in a fit of rage, but then later regret his actions. He would probably start to question his own judgment in friends and become more guarded in future relationships. In some fanfiction, he might even go on a journey to find out why they betrayed him, determined to get to the bottom of it and either forgive them or get revenge.
Ash could respond in different ways. He might try to confront his friends immediately to get an explanation. If the betrayal is really bad, he could cut them off completely and focus on his own goals, like becoming a better Pokémon trainer. He would also probably have some internal struggles, as he has memories and experiences with these friends that he can't just forget.
He might be initially shocked. Harry has always trusted his friends, so when the betrayal happens, it would be like a bolt from the blue. He could stand there speechless for a moment, unable to believe what has just occurred.
At first, Harry would be in disbelief. He would think it was some sort of joke or misunderstanding. But as the reality sets in, he would become a bit of a loner. He'd still have his sense of right and wrong, so he might continue to fight against the dark forces on his own, all the while hoping that his friends would come to their senses and realize they made a huge mistake in betraying him.
If his friends continue to ignore him, Harry could become sad and withdrawn himself. He might start to isolate more, spending time alone in places like the Forbidden Forest or an empty classroom. He might also lose his confidence in himself, thinking that he is not worthy of their friendship. In his loneliness, he could focus more on his studies or some solitary magical pursuits, trying to distract himself from the pain of being ignored.
He would be heartbroken. Naruto values friendship above almost everything. At first, he might not believe it and try to reason with his friends.
He probably feels extremely hurt and confused. Betrayal by friends is a very painful experience, and Bell might question his own judgment in trusting them in the first place.
At first, Naruto might be really confused. He's used to thinking of his friends as his support system, so when they turn on him, he doesn't understand why. He'll probably try to talk to them, to ask what he did wrong. If they don't listen, he could become really sad and withdrawn. Maybe he'll start training even harder, using the pain of being rejected by his friends as motivation to become stronger.
Ash would probably be initially shocked. He has always trusted his friends, so the idea of betrayal would be hard to swallow. He might try to talk to them, to find out why they are doing this.
Sonic usually feels a great deal of shock and confusion at first. He can't believe that his friends would turn on him. Then he might feel anger, especially if he discovers that they were misled or acting out of unjust reasons.
Well, Sakura could also try to understand why her friends betrayed her. She might be the kind of character who looks for the root cause of the problem. She could approach her friends, not to forgive them right away, but to have a conversation. By doing this, she hopes to find out if there was some misunderstanding or if they were forced into betraying her.