Sometimes, it might be an ancient monster that has a grudge against the lineage that Jason and Piper are from. So it targets them specifically, seeing them as a threat to its own existence or plans.
There could also be a group of rogue demigods who envy Jason and Piper's status or powers. They might create all sorts of troubles for the couple, from stealing their weapons to spreading lies about them among other demigods.
It could be a powerful sorcerer who wants to use Jason and Piper's powers for his own evil deeds. This sorcerer might try to manipulate them into doing his bidding or separate them to weaken their combined strength.
One interesting plot could be Jason and Piper going on a quest together. They might face various monsters and challenges that test their relationship and individual strengths.
In some fanfictions, it could be the old enemies of Percy like the Titans or Giants who see his relationship with Piper as a weakness and try to use it against him. For example, they might try to kidnap Piper to lure Percy into a trap.
The main characters are obviously Jason and Piper. Jason is the one who is unfaithful in these stories. Piper is the one who is being cheated on. Their relationship and the fallout from Jason's actions are the central focus of this type of fanfiction.
One common antagonist could be a power - hungry doctor in the hospital who sees Jason and Robin as threats to his own rise in the ranks. He might try to sabotage their work or spread false rumors about them. Another could be a business tycoon who wants to take over the General Hospital and will do anything to get rid of those who oppose him, like Jason and Robin. A third could be an ex - lover of either Jason or Robin who is jealous of their relationship and tries to break them up.
One key element is their relationship dynamic. Piper's strong - willed nature and Jason's leadership qualities often clash and then reconcile in interesting ways. Another is the magical elements that are often intertwined with their story, like the use of demigod powers.
Jason and Piper are the main characters. Jason is a hero who has a lot of responsibility on his shoulders. Piper is his partner, and she brings her own set of skills to the table. Their relationship is also a key aspect of the fanfiction as they support and rely on each other in different situations.
In fanfiction, Jason and Piper often have a complex relationship. They might start as friends, gradually realizing their deeper feelings. Their adventures together can strengthen their bond, and they support each other through various challenges. For example, in some fanfics, they face mythical creatures side by side, which makes them rely on one another more.
One possible antagonist could be an ancient and powerful Titan who has escaped from Tartarus. This Titan is bent on revenge against the gods and sees Percy and Jason as the key to getting to the Olympians. He uses his dark magic to create all sorts of disasters and tries to turn Percy and Jason against each other. Another could be a group of rogue demigods who have turned to the dark side. They envy Percy and Jason's status and friendship and try to undermine them at every turn.
It could be some new characters created specifically for the fanfic. Maybe a group of rogue ninjas who are after a powerful artifact that Naruto and Hinata are also trying to protect.
Frieza and his minions would be obvious antagonists. They are the ones who brought so much destruction to the Saiyans. In a fanfic, they could be trying to finish off any remaining Saiyans, including the Saiyan princess and Bardock if he's still alive.