Ginny could be a bit shy about it. She might read the letter over and over again in private, blushing each time. Then she would write a response letter, expressing her own feelings for Harry in a more reserved but still very loving way.
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.
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.
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.
If the letter contains bad news or a problem, Hermione would likely be very concerned. She's a problem - solver, so she'd start thinking about solutions right away. For example, if Harry tells her about a new danger in the wizarding world, she'd start making plans to protect themselves and others. Her reaction would be one of determination and focus.
He might be suspicious at first. Given their different houses and the general tensions between Slytherin and Gryffindor, Harry would be cautious about Blaise's motives.
Ginny might initially be heartbroken. She could retreat into herself, feeling betrayed and confused. In some fanfictions, she may cut off all contact with Harry immediately.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.
Ginny could be heartbroken. She might have expected to marry Harry after all they've been through together. She could initially go into a state of shock and disbelief, not understanding why Harry has refused.