Bella is obviously one of the most popular. Since she is the main character in the original series, in fanfiction she often remains the central focus. Her relationships with the Cullens, especially Edward, are often explored in great depth.
One interesting plot could be Bella developing a new power while living with the Cullens. For example, she might gain the ability to communicate with animals like some of the other vampires. This would add a new dynamic to her relationship with the Cullens and also pose new challenges and adventures as they try to keep it a secret from the outside world.
It depends on how you define 'popular'. There are a lot of Twilight fanfictions out there, and this particular theme might not be the most common. However, within certain subsets of the fanfiction community, it could be quite popular. For example, fans who are interested in exploring power dynamics and the darker side of the characters might be drawn to this type of fanfiction. It also might appeal to those who want to see Bella in a more vulnerable position and how she would react to being bullied by the Cullens, which is a departure from the typical story where she is often the center of their affections.
In the original Twilight story, Bella is initially attracted to the Cullens, especially Edward. But in this fanfiction where she starts to hate them, it's a complete contrast. It shows a different side of Bella's character, one that is more critical and less forgiving. In the original, she is more likely to overlook their flaws because of her love for Edward. Here, she focuses on the negative aspects.
Definitely Bella and the Cullens. But also, there could be some minor characters from Forks. Maybe Charlie, Bella's father, could unknowingly play a part. He might notice Bella acting strange and start to investigate, which could add some tension to the story. And perhaps some of the other humans in Forks that Bella has befriended could also be involved, like Angela or Jessica, who might see Bella doing strange things and start to suspect something about the Cullens.
One possible plot could be that Bella discovers the Cullens are homeless for some reason, perhaps they've been run out of their home by a new vampire threat. Bella then uses her connections in the town and her knowledge of the area to find them a safe place to stay. She might also help them gather resources, like getting blood from a hospital in a discreet way.
Her relationship with Jacob would likely change. Since she hates the Cullens, she might turn more towards Jacob for support. They could become closer as she sees Jacob as an alternative to the Cullen world. With her human friends, she might try to distance herself from any connection to the Cullens. So, she could be more secretive or withdrawn from them as well to avoid any talk about the Cullens.
She might start by giving them the silent treatment. Just ignoring them completely when they try to talk to her, which would be a very human way of showing anger.
Well, it could be due to the way the Cullens interact with the other people in Forks. If Bella notices that they are being manipulative or using their powers in a way that she doesn't like to influence the townspeople, that could make her start to hate them. Also, if there is some kind of power struggle within the Cullen family that starts to affect Bella negatively, like she gets caught in the middle of it or she sees how it's causing harm to those she cares about, she might turn against them.
It could be that the fanfiction writer took a different approach to the story. In some fanfictions, the Cullens could be involved in situations that go against what Bella stands for. Perhaps they are written to be more secretive and untrustworthy, leading to Bella's hatred.