Perhaps a human character who has an unhealthy obsession with Bella. Maybe someone who has always been in the background, like a shy classmate who has developed an extreme fixation on her and is now stalking her to be closer to her in a very wrong way.
There's also the possibility that it could be a werewolf from a rival pack. They might see Bella as a threat to their territory or a way to get to the Cullens, so they start stalking her to gather information or to try and scare her away.
In some fanfictions, the stalking situation might be part of a larger, more complex supernatural plot. For example, the stalker could be a vampire with a strange vendetta against Bella that has to do with her relationship with Edward. This could involve Bella having to use her wits and any latent powers she might have to protect herself.
It could be a character named Jake. He's a very competitive Bakugan brawler who sees Bella as a threat because she's quickly rising in the ranks. He's always trying to one - up her in battles and spread rumors about her to make other brawlers not trust her.
Edward could be one of her allies. He could use his vampire abilities such as super - speed and strength to assist Bella in dangerous situations. His centuries - old knowledge could also prove invaluable in understanding the motives and actions of the CIA's enemies. Another ally could be Jacob. His shape - shifting abilities would give the team an extra edge in combat or in covert operations. He could also use his connection to the werewolf pack to gather information from areas that the CIA might not have easy access to.
Perhaps it's a human with a very strong psychic ability. He is drawn to Bella because he can sense her connection to the supernatural. Their relationship could be based on mutual understanding of the secrets they both hold, and Bella might find his normalcy in some ways refreshing compared to her previous relationships with vampires and werewolves.
In fanfiction, Bella might be the Volturi's princess because of a misinterpreted prophecy. The Volturi could believe that Bella is the key to something great, like ruling over all vampires or finding a way to live in the open without fear. So, they give her this princess - like status. Also, it could be due to her relationship with Edward. If the Volturi see Edward as a threat or an asset, they might use Bella to manipulate him, and making her a princess is part of that strategy.
Maybe it's Loki. Loki is known for his mischief and cunning. His complex personality would add a lot of drama to the relationship with Bella. He could bring a whole new level of chaos to the Twilight universe.
It could be a new vampire. Maybe a vampire from a different coven who comes to Forks and is immediately attracted to Bella's unique scent and her strong - willed personality.
The fact that Bella Swan is a Teretto fanfiction could be a really interesting concept. It might bring in elements related to Teretto into the world of Bella Swan. For example, if Teretto has a certain culture or set of values in this fanfiction, Bella might start to adopt some of those ideas. It could also create new relationships for her with other characters who are also involved with Teretto in some way.
A loving couple could be the potential adopters. They might be a married couple who have always wanted to have a child but couldn't have one biologically. So, they decide to adopt Bella. They have a stable home and enough resources to take care of her.
Perhaps a new character could be introduced as the hero. A wizard or a powerful human with some extraordinary skills. This new character could be someone who has been observing Peter's actions from afar and decides to step in to save Bella when the time is right.