The main characters are Harry and Fleur, no doubt. However, depending on the plot of the fanfiction, characters like Voldemort or his Death Eaters could also be important. If the betrayal is related to the war against Voldemort, they could be the ones who manipulate Fleur into betraying Harry. Additionally, members of the Weasley family might be involved. For instance, if Fleur has a relationship with Bill Weasley, his family's reaction to her betrayal of Harry could be part of the story.
One possible plot could be that Fleur betrays Harry in a Triwizard Tournament - related task, perhaps by secretly helping the other champions. Another might be that Harry discovers Fleur has been passing information about the Order of the Phoenix to the wrong side. Maybe Fleur betrays Harry's trust regarding a personal secret, like his feelings for Ginny, and uses it to her advantage in some way.
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.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
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.
Another possibility is that Fleur, in a bid to gain more influence in the magical world, uses Hermione's research on some ancient magic without permission. When Hermione finds out, she realizes Fleur has been using her all along, and this is the ultimate betrayal. Hermione then has to find a way to expose Fleur's actions and clear her own name.
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.