There could also be new mythical creatures created specifically for the fanfiction. These creatures might be sent by an unknown force to disrupt Percy and Piper's relationship. They could be causing chaos in the demigod world and making it hard for Percy and Piper to have a normal relationship.
Sometimes it might be other jealous demigods within the camp. They might spread rumors or create obstacles for Percy and Piper's relationship. They could be envious of the attention Percy and Piper get or simply not approve of their relationship for some reason.
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.
One of the main antagonists could be the Titans that were banished to Tartarus. They are powerful and hold grudges against the demigods. For example, Kronos' influence might still be felt in Tartarus, and his minions could try to stop Percy and Piper.
There are some. However, the quality can vary greatly. Some might focus more on the romantic aspect, while others might incorporate more of the action and adventure elements from the Percy Jackson universe. It really depends on what you're looking for in a fanfiction. You might want to read the reviews or summaries first to find the ones that seem most appealing.
Another plot could involve Piper using her charm - speak ability to help Percy in a difficult situation, and this act deepens their bond. Maybe they are trying to save a group of half - bloods from a powerful monster, and their combined skills along with their growing love make it possible.
It could be an old enemy from Greek mythology that has been resurrected. For example, a powerful Titan who has a vendetta against the demigods, especially Percy and Annabeth.
Maybe a character from their past who has a hidden grudge. Say, someone they defeated in a previous battle who has now found a way to get revenge. This person could pretend to be their ally again and then stab them in the back when they least expect it.
Well, in some fanfictions, Voldemort or his remaining Death Eaters could be antagonists. They might see Harry and Hermione's relationship as a threat to their plans. For example, if they think that a united Harry and Hermione are more powerful and could foil their dark schemes more easily.
Well, a plot could involve Piper using her charmspeak abilities to accidentally make Percy fall for her at first. But then as they spend more time together, she realizes she actually has real feelings for him. They then have to deal with the reactions of their friends, like Annabeth might be a bit jealous at first. And they also have to balance their relationship with their duties as demigods, constantly being called away on new missions.
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.
Apollo himself could be seen as an antagonist in a way, because of his possessive behavior towards Percy. His actions that restrict Percy's freedom and independence are antagonist - like. However, in some cases, other characters who oppose Apollo's possessiveness might also be antagonists from Apollo's perspective. For example, if Percy has a close friend who tries to protect him from Apollo, Apollo might view that friend as an enemy.
Maybe it's the Titans who are still lurking in Tartarus and don't want Percy and Zoe to succeed in whatever mission they have. The Titans could see them as a threat to their potential future plans of rising again.