Well, it's the loyalty and trust they have for each other. In many fanfictions, Edward and Bella as best friends are always there for one another. Whether it's facing bullies at school or dealing with family drama, they know they can count on each other. This unwavering support in a non - romantic context is really endearing and makes the relationship very appealing to readers.
In fanfiction, their best - friend relationship is appealing because it often shows a different side of their personalities. For example, they might joke around and be more carefree as friends compared to the intense romance in the original story. It gives a fresh perspective on their characters. Also, their friendship can be a source of growth for both of them, helping them become better individuals.
The fact that they can understand each other on a deep level. They both have their own struggles, like Bella's shyness and Edward's self - imposed isolation in some stories. As best friends, they break through those barriers and become a support system for each other.