The main antagonists could be other vampire clans that are jealous of the Cullens' power and see Jasper and Bella's mating as a way to create an even stronger bond within the family. These clans might try to break them apart or cause trouble for them. Also, there could be a human hunter who discovers the existence of vampires through Jasper and Bella and decides to hunt them down.
Some fanfictions might feature a vampire who was once rejected by Jasper or has a past with him. This vampire could see Bella as a way to get revenge on Jasper. They would try to come between them, perhaps by spreading lies about Jasper to Bella or causing problems for them in the vampire community. And, of course, the Volturi are always a potential source of conflict as they are the ones who enforce the vampire laws.
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.
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.
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.
The two main characters are Jasper and Bella. Jasper is a vampire who has seen a lot in his long undead life. His past experiences shape his character and how he approaches his relationship with Bella. Bella is the human who becomes his mate. She has her own insecurities and strengths. In the fanfiction, their relationship is explored through their different personalities, with Jasper's vampire nature and Bella's human nature often creating interesting situations and storylines.
Sometimes, other werewolves who don't accept Bella as Jacob's mate could be the antagonists. They might think that a human - werewolf relationship goes against their pack's rules or beliefs.
Often, other vampires or supernatural creatures could be the main antagonists. For example, vampires who are jealous of Damon's newfound mate might try to drive a wedge between them. Maybe there are some werewolves who see Bella as a threat to their territory because of her relationship with Damon.
Perhaps there are demons or dark forces that are trying to disrupt the union between Bella and the God of War. These antagonists could be bent on causing chaos and destruction in their world, seeing their relationship as a symbol of hope that they want to destroy.
The Volturi could be main antagonists. Since they have strict rules about vampires and humans, they might see Bella and Jasper's relationship as a threat to their order.
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 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.
One common theme is the struggle against their different natures. Jasper has his violent past as a vampire, while Bella is a human. Fanfictions often explore how they deal with this difference.