Alice might be the one. Given her ability to see the future, if she saw something bad happening to Bella but didn't warn her because it would disrupt the Cullens' plans, that would be a major betrayal. She could have known about a danger approaching Bella but chose not to tell her in order to protect the family's interests instead.
Another possibility is that it could be a new character introduced in the fanfiction. This new character could be someone who gains Harry's trust quickly but then betrays him for personal gain or because they are secretly working for an enemy. For example, a new student at Hogwarts who seems friendly at first but is actually a spy for a dark wizard.
It could be another vampire clan. Maybe they are jealous of the Cullens' interest in Bella or they think that Bella's presence among the Cullens will disrupt the balance of power among vampires. So they try to get Bella from the Cullens, making them the main antagonists.
Well, perhaps Bella discovers that the Cullens have been lying to her all along about their true intentions. Maybe they were using her for some ulterior motive, like protecting their vampire secrets from other threats in a way that betrays her trust.
There could be other vampire clans that see the Cullens' camping trip with Bella as a threat or an opportunity. They might try to disrupt their time in the woods. Another possibility is that some shape - shifters in the area don't like the presence of vampires and decide to cause trouble.
Obviously, Bella and the Cullen family are the main characters. Bella is the adopted one. The Cullen family includes Edward, who has a special relationship with Bella, Carlisle who is like the father figure, Esme the motherly figure, and the other Cullen siblings like Alice, Jasper, Rosalie, and Emmett.
Obviously, Bella is one of the main characters. She is the one being adopted into the Cullen family.
Obviously, Bella and the Cullens are the main characters. Bella is the human who enters the world of the Cullens. And the Cullens include Edward, who has his own complex personality and abilities as a vampire. Then there's Alice, who might be able to see the future and plays an important role in Bella's interactions with the family.
Her father, Charlie, might try to rescue her. He loves Bella deeply and would be very worried when she goes missing. He could start investigating and find out that the Cullens are involved.
It could be an enemy who has infiltrated his team disguised as a friend. For example, in a crossover with Fullmetal Alchemist, an alchemist with the ability to change appearance might pretend to be on Kakashi's side but then betray him.
It could be his friend, like Mitsuki. Mitsuki has his own secrets and goals, and in some fanfiction, he might be forced to betray Boruto for the sake of some greater good in his mind. For instance, if he has to choose between following Orochimaru's orders and staying loyal to Boruto.