This is a Twilight and Vampire Diaries Crossover so it involves aspects of each series of book.
Bella is not human, so what is she exactly? She's a Salvatore.
When she was younger, her brothers fell in love with a girl named Katherine Pierce and they picked that girl over their own sister. Who in their right minds would do that?
After her brothers, Damon and Stefan die she disappeared and no one knows what happened.
So what happened? Eventually Bella ended up in Forks,Washington and pretends to be human and may have finally found her place in life.
This is the story of her life after the death of Stefan and Damon.