In some cases, there could be a rogue vampire with a vendetta against the Cullen family. This vampire might target Alice and Jasper specifically because of their unique abilities and importance within the family. They could try to disrupt their relationship or harm them in other ways.
Sometimes, in fanfiction, there are also human antagonists. Maybe a group of hunters who figure out that Alice and Jasper are vampires and try to hunt them down. They could be using advanced technology or ancient vampire - hunting techniques, which would make it a real challenge for Alice and Jasper to evade them.
The Volturi could be antagonists. They are the powerful vampire coven that enforces vampire laws. Since Bella's transformation might not follow the normal protocol, they could see her as a threat to their order. They might come after her to either destroy her or bring her under their control.
In some Bella and Jasper military fanfictions, the main antagonists could be rival military groups. These groups might have different ideologies or goals and be in direct competition or conflict with Bella and Jasper's unit. For example, they could be fighting over territory or resources. Another type of antagonist could be a power - hungry military leader within their own ranks who is willing to sacrifice the well - being of the soldiers for personal gain.
Often, the Volturi can be antagonists. They might not approve of Jasper and Bella's relationship for some reason, like they think it goes against vampire laws or traditions. Another possible antagonist could be a rogue vampire who has a vendetta against the Cullens and sees Jasper and Bella's relationship as a way to get to them.
The main characters are obviously Jasper and Alice. Jasper is often depicted as a complex character with a troubled past, while Alice is known for her clairvoyance and positive energy.
It could be an ex - partner of either Leah or Jasper. For instance, if Jasper has an ex - girlfriend who is jealous of Leah and the new baby, she might try to cause trouble between them. Maybe she spreads false rumors about Leah to make Jasper doubt her.
Often, the Volturi can be antagonists. They have strict rules about vampire relationships and activities. If Bella and Jasper's relationship goes against their laws, the Volturi would oppose it. They might see it as a disruption to the vampire order they uphold.
One interesting plot could be Alice and Jasper facing a new and powerful vampire coven that threatens their family. They have to use their unique abilities, Alice's visions and Jasper's ability to control emotions, to outwit the new threat.
One major theme is acceptance. Jasper has to accept Alice's human nature with all its weaknesses and Alice has to accept Jasper's vampiric nature. Also, the theme of danger often comes up. Since Jasper is a vampire, there are threats from other vampires or vampire - related elements that endanger Alice.
Bella could also be a significant character. She might be the reason for their meeting in some stories. For example, if Bella is in trouble, Alice and Jasper could come together to protect her. Her human nature can contrast with the vampire nature of Alice and Jasper, creating an interesting backdrop for the story.
It could be other vampires who are jealous of their unique relationship. Maybe there are some traditionalist vampires who don't accept this new kind of mating bond among them.