In this fanfiction, the vampire could be an old acquaintance of Fleur from her Beauxbatons days. He was once a student there who got turned into a vampire later. Harry gets involved when he notices some strange happenings around Fleur. The three of them then get caught up in a mystery that involves a powerful dark magic item. They must work together, with Harry using his Gryffindor courage, Fleur her Veela charm, and the vampire his immortality - related skills to solve the mystery and keep the item from falling into the wrong hands.
For Harry, you could focus on his sense of justice. Maybe he's initially wary of the vampire but his experiences with Voldemort have taught him not to judge too quickly. Fleur can be developed by highlighting her Veela allure and how it affects the vampire. The vampire could be developed by giving him a tragic backstory. For example, he was turned against his will and is trying to find a way to reverse it. This could create an interesting dynamic between the three characters.
Often, Luna Lovegood can be found in such fanfictions. Her unique and dreamy nature could bring a different dynamic to the harem. Neville Longbottom might also be involved in some way, perhaps as a friend who supports the harem and helps them out in times of need. There could also be some foreign exchange students or magical creatures in human form that end up in the harem for various reasons.
At first, their relationship might be a bit strained. Harry is used to his own space and friends, and Fleur has her own circle. But as they live together, they'll start to respect each other's differences. For example, Fleur might be more flamboyant in her magic use, while Harry is more practical. They'll start to see the value in each other's approaches and become closer friends.
One possible reason could be that Fleur sees Harry as a powerful and important figure. In the wizarding world, Harry is the famous Boy Who Lived. Fleur might be possessive because she wants to be associated with his fame and power. Also, his bravery and skills could be attractive to her, making her want to have him all to herself in the fanfiction world.
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.
Often, Harry's sense of responsibility grows. He starts to see the marriage contract as not just a burden but also an opportunity to create a new life for himself and Fleur. Fleur's personality may develop in a way that she becomes more protective of Harry. For example, when they face opposition from the wizarding community about their contract, she stands up for them. She also might help Harry to overcome his insecurities about being in a relationship with someone as glamorous as her.
At first, they might be at odds with each other. Fleur could think Harry is too impulsive, while Harry might find Fleur a bit too haughty. But then, when they are forced to work together on a mission, they see the good in each other. Fleur realizes Harry's selfless nature, and Harry sees Fleur's intelligence and determination. This new - found respect leads to a romantic relationship and ultimately marriage.
In many fanfiction interpretations, Fleur's response would depend on how the author has developed her character. If the author has portrayed her as more open - minded and adventurous, she might quickly accept Harry's invitation with a smile. But if she is depicted as more cautious or reserved, she could decline politely, perhaps stating that she already has other plans or that she doesn't think it would be appropriate given the inter - school dynamics.