Edward would likely be the most affected. He's in love with Bella. Her leaving would be like losing his whole world. He might become withdrawn and even more brooding than before.
In this fanfiction, it might be that Bella has received some sort of premonition. She could have seen a future where staying with the Cullens leads to their destruction. Out of love for them, she chooses to say goodbye. She might plan to go far away, change her identity, and live a life away from the supernatural world that has been both alluring and dangerous for her.
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.
Edward is also a fascinating character. His struggle with his vampire nature and his growing feelings for Bella can be explored in more depth in the fanfiction. Maybe we'll see new aspects of his character, like his relationship with his vampire siblings in the context of Bella being in their lives.
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.
In the 'the cullens kidnap bella fanfiction', it could be that the Cullens have some hidden motives. They might believe that Bella has a special power or connection that they need to study or keep safe. So they take her against her will, which then leads to a complex relationship dynamic between Bella and the Cullens as she tries to figure out what they really want.
In the 'bella is kidnapped by the cullens fanfiction', one possibility is that Bella is kidnapped to be part of some vampire ritual. However, she refuses to cooperate at first. The Cullens then have to try and convince her that it's not as bad as it seems. This could involve showing her their more human - like qualities, like their love for art and music. As Bella spends more time with them, she may start to see them in a different light and even develop feelings for one of them.
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.
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.
Obviously, Bella and the Cullens are the main characters. Bella is the human girl who enters their world. And the Cullens include Edward, who has a special connection with Bella. Then there's Alice, who might be able to see the future and could play an important role in Bella's interactions with the family. And of course, Carlisle, the head of the Cullen family.
In fanfiction, when Bella reunites with the Cullens, there could be a lot of emotional exchanges. Maybe there are warm hugs and tears of joy. They might immediately start catching up on all that has happened since they were apart. The Cullens could express their relief at having Bella back, and Bella could feel a sense of coming home.