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.
Perhaps in the fanfiction, Harry and Fleur are in a relationship, and Fleur betrays Harry by getting involved with someone else, like Bill Weasley, in a more devious way than in the original story. It could also be that Fleur betrays Harry's trust in their friendship by using a spell on him to steal his magical abilities or knowledge for her own gain. Or, there could be a political betrayal where Fleur, who may be representing Beauxbatons in some post - war wizarding world politics, goes against Harry's ideas and plans, which he thought she supported.
In some fanfictions, it could be that Fleur is under the Imperius Curse and is made to betray Harry. For example, she might be forced to lead him into a trap set by Voldemort. Or, it could be a situation where Fleur is jealous of Harry's relationship with Hermione (in some fanfic interpretations) and betrays him to get Hermione out of the picture. There could also be a plot where Fleur betrays Harry for the sake of her family's reputation or safety, by turning him in to the Ministry when they start to target Muggle - borns.
One possible plot could be that everyone believes false rumors about Harry being a dark wizard. His friends from Hogwarts, including Ron and Hermione, turn their backs on him. Harry then has to find a way to prove his innocence all alone, perhaps by going into hiding and secretly investigating who spread the lies.
In some fanfictions, it could be a misunderstanding that spirals out of control. Hermione and Ron misinterpret one of Harry's actions. For instance, Harry might be trying to protect them in a secret way, but it looks like he's up to something bad. Their lack of trust then grows, and they end up betraying Harry, perhaps by not coming to his aid when he really needs them, which could have serious consequences for Harry.
Well, perhaps in the fanfiction, it shows that Harry's parents made a deal with Voldemort before their deaths. They promised to hand Harry over in exchange for something, like power or protection for themselves. But then things went wrong and they ended up dying, still leaving Harry feeling betrayed as he discovers this truth later. It could also be that his parents were actually working against the Order of the Phoenix all along and Harry was just a pawn in their real plan.
In some fanfictions, it could be that Harry discovers a dark secret about the wizarding world that his friends don't believe or are too afraid to accept. His friends, thinking he has gone mad, side with the established order. They then betray Harry by reporting his so - called 'crazy ideas' to the Ministry of Magic, leading to Harry being hunted.
One possible plot could be that Tails is betrayed by a close friend who becomes jealous of his intelligence and skills. For example, Sonic might start to feel overshadowed by Tails and in a moment of weakness, turn against him. This could lead to Tails having to fend for himself and prove his worth all over again.
One interesting plot could be their cultural differences. Harry, being from a more typical British wizarding family, and Fleur, with her French Veela heritage. They might have to navigate different magical traditions and family expectations. For example, Fleur's family might have specific Veela - related customs during holidays or family gatherings that Harry has to get used to.
One possible plot could be that Boruto is betrayed by his friends due to a misunderstanding. For example, they might think he has turned to the dark side because of some false evidence planted by an enemy. Another could be that Boruto is betrayed by the village he loves. Maybe the village leaders suspect him of a crime he didn't commit and turn against him.
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.
In some 'Wei Wuxian betrayed fanfiction', it might be that he is betrayed by a family member or a disciple he had trained. For example, they might side with his enemies in a battle, believing the lies told about Wei Wuxian's so - called 'evil ways'. This would make Wei Wuxian feel extremely hurt and isolated, having to face the challenges alone.
One possible plot could be that Ash discovers Anabel has betrayed him by siding with a rival trainer in a crucial tournament. This could lead to a rift in their friendship and Ash having to find a way to overcome this setback and prove himself without her support.