After the initial shock and anger, Harry might start to feel sad and a bit lost. Ginny has been an important part of his life, and this 'betrayal' would leave a big hole. He could struggle with his identity as well, since Ginny was part of his journey to find a normal life after all the battles. Depending on the fanfic, he could either forgive Ginny in the end if he discovers there were mitigating circumstances, or he could hold a grudge and cut her out of his life completely.
Harry might become angry and feel a sense of betrayal. He could isolate himself from others, especially the Weasley family. In some fanfics, he might even start to question all of his relationships, thinking that if Ginny could betray him, then who else could. He could also be determined to find out the truth behind Ginny's actions, so he would start investigating on his own, not trusting anyone else's version of events at first.
He could be initially shocked. Harry, who has trusted Ginny deeply, would find it hard to believe at first. He might go through a period of denial, trying to find excuses for Ginny's actions.
He could also feel a great sense of anger. After the shock wears off, he might lash out at Ginny, demanding an explanation for her actions. His anger could be so intense that it might cloud his judgment for a while.
He might be extremely angry and confused. After all, parents are supposed to be the ones you can trust the most. He could start to question everything he knew about his family and himself.
Ginny might be overjoyed. She could blush deeply and then give Harry a big hug, telling him she feels the same way.
She might blush. Since Ginny has feelings for Harry, his stare could make her feel self - conscious and her cheeks turn red.
Harry would probably be heartbroken. He trusts Ginny deeply, so her betrayal would be a huge shock. He might initially be in denial, not wanting to believe that Ginny could do such a thing.
At first, Harry would probably be in shock. He has always considered Hermione one of his closest friends, so her betrayal would be a huge blow. He might not believe it at first and try to reason with her.
Ginny would most likely be extremely happy. She has deep feelings for Harry, so his proposal would be a dream come true for her.
On the other hand, Ginny might secretly be a bit flattered that Harry is so possessive over her. But still, she would probably try to stop the situation from getting worse and make Harry understand that he can't just go around punching people because of her.
Alternatively, Ginny could be very mature about it. She might respect Harry's decision, even though it hurts. She could choose to step back and give Harry the space he seems to need while secretly hoping that he will change his mind in the future.
Harry might be initially shocked and heartbroken. He trusted the Weaslys like family, so the betrayal would hit him hard. He could become more withdrawn and distrustful of others.