Who could be the main antagonists in an issei half vampire fanfiction?

Other vampires might be his main antagonists. They could see Issei as a threat, perhaps because he's a new vampire with different ideas or powers. Or they might want to use him for their own gain.

It could be an old enemy of the group who has a personal vendetta against Issei. This antagonist might be jealous of Issei's power or popularity within the group and plots to turn everyone against him. They could spread false rumors or create false evidence to make it seem like Issei has betrayed the group.

The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.

In this fanfiction, the Church or religious authorities in Camelot could be the main antagonists. Vampires are often associated with the devil or evil in religious beliefs. So, they might see Merlin's transformation into a vampire as a sign of him being corrupted by dark forces. They would try to exorcise or destroy him, not understanding that he's still the same Merlin at heart, just with a different form and new powers.

Perhaps it could be a group of bullies from Issei's school days. They used to pick on him and now he's coming back to make them pay.

Another vampire, an older and more powerful one, could be an antagonist. This vampire might see Percy as a threat to his power or territory. Maybe he wants to either kill Percy or make him submit to his rule.

There could also be a powerful vampire hunter. This person might have a personal vendetta against vampires in general or specifically against Ichigo. They could use all kinds of weapons and tactics to try and kill him.

It could be a dark sorcerer who has been studying the secrets of angels and vampires. He discovers Naruto's existence and wants to use him as a key ingredient in a dark spell. The sorcerer is power - hungry and believes that by harnessing Naruto's half - angel half - vampire essence, he can open a portal to a dimension filled with dark magic and bring chaos to the ninja world. He would be constantly devising traps and sending minions after Naruto to capture him.

In certain fanfictions, fallen angels may act as antagonists. They could be trying to disrupt the relationship between Akeno and Issei or simply cause chaos in the devil world where they live.

The vampire Authority could be antagonists. They might see Sookie as an abomination since she was a fae - human before being turned into a vampire. They could try to eliminate her to maintain the 'purity' of the vampire race according to their own rules.