He might become withdrawn. Harry has been through a lot in his life, and being heartbroken by Fleur could make him retreat into himself. He may avoid his friends and isolate in his dormitory at Hogwarts.
Another possibility is that Harry would try to win Fleur back. He's a determined person. He might start to investigate why Fleur has broken his heart. If it's a misunderstanding, he'll do everything he can to clear it up and get her back. He could study more advanced magic to try and break any spell or curse that might be affecting Fleur.
At first, Harry would likely be shocked and angry. He's used to being in dangerous situations, but being kidnapped by Fleur, someone he thought was an ally, would be a big betrayal. He'd probably try to fight back immediately, using his magic if he could.
Well, perhaps Fleur rejects Harry for some reason. Maybe she's still in love with Bill and tells Harry she can't be with him. This would leave Harry heartbroken as he might have developed deep feelings for her.
Sometimes, Harry might use the rejection as motivation. He could focus more on his training or other important tasks at hand, like fighting against the Dark forces. He realizes that he has bigger things to worry about than a failed romantic pursuit, and this rejection could fuel his determination to prove himself.
In some fanfictions, Harry Potter might help Fleur by using his knowledge of magic. For example, if Fleur is in a dangerous situation during a wizarding tournament task, Harry could share some of his unique insights on how to overcome the magical obstacles, having experienced similar challenges himself.
Hermione would probably be a bit skeptical at first. She might worry that Harry is rushing into things. But once she sees how happy they are, she would be supportive. Ron, on the other hand, would be more accepting from the start, especially if there are good feasts at the wedding.
She might be shocked at first. Fleur is used to beauty and perfection, and Harry's scars are a sign of his many battles and hardships. So she could show a moment of wide - eyed surprise.
Well, Fleur is a complex character. When she finds out about Harry's abuse, she might initially be in disbelief. But once it sinks in, she would feel a deep sense of sympathy. Fleur might use her own experiences to relate to Harry. For example, she has faced prejudice in the wizarding world due to her Veela heritage. She could draw on that strength and determination to help Harry. She may offer him a place to stay away from his abusers, and use her magic to create a safe and comfortable environment for him. Also, she could start training Harry in some defensive magic so that he can protect himself in the future.
He might be a bit shy and stutter out a simple 'you're welcome'. Harry is often modest in such situations.
Harry might be a bit flustered at first. He's not used to the attention from someone like Fleur. He could be shy and a bit unsure of how to handle it.
As time goes on, Harry could start to explore what it means to 'please' Sirius. He might recall their past relationship, the things Sirius likes and dislikes, and try to use that knowledge to his advantage. He may also feel a sense of loyalty towards Sirius that, despite the odd circumstances, makes him more accepting of the situation. For example, he might remember Sirius' love for adventure and try to engage in some magical adventures with him in a way that 'pleases' him.