In some versions, the werewolves could be antagonists. Since they have an uneasy relationship with the vampires in the Twilight universe, they might not be happy about Bella being with Jasper. They could view it as a betrayal of sorts, especially if they had some sort of alliance or understanding with Bella in the past. Their interference could create a lot of conflict in the fanfiction.
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.
Some other vampires who are jealous of Jasper or have a vendetta against him could be antagonists. They might try to come between Bella and Jasper, either out of envy or because they want to use Bella to get to Jasper. In fanfiction, there could also be new characters created specifically to be antagonists, like a powerful vampire who wants Bella for himself and sees Jasper as an obstacle.
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.
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.
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.
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.
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.
Yes, there are quite a few. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for the specific tags related to 'bella is jasper and peter's mate' and you'll find a variety of stories with different takes on this concept.
The main characters are clearly Harry and Jasper. Harry is likely to be a character with his own set of traits, perhaps brave and curious. Jasper, on the other hand, may have characteristics that complement Harry's. They are the central focus as they are the ones in the mate relationship.
It could be the Titans who escaped from Tartarus and are seeking revenge. They might see Percy and Apollo's relationship as a threat to their plans of overthrowing the Olympians. Another possibility is some jealous gods or goddesses who don't like the idea of Percy and Apollo being mates.