Sometimes, it could be characters who have been corrupted. Maybe a former ally who has turned against Percy due to some outside influence. This could add an interesting layer of betrayal to the story. The reasons for their corruption could range from being influenced by dark magic to being manipulated by a more powerful evil force.
It could be the traditional villains from the original story. Maybe some of the old enemies of Percy who have a vendetta against him. For example, the Titans or some rogue demigods who oppose him for power or personal reasons.
There might be new characters created specifically for the fanfiction. They could be powerful and cruel beings from some hidden part of the fictional world. These antagonists could be motivated by jealousy towards Percy's status or abilities. They see torturing him as a way to assert their own dominance or to gain something from him, like his powers or secrets.
It could be the traditional villains from the original story, like the Titans or some powerful monsters. They might torture Percy to get information or just to cause chaos.
Another option is a group of demons who have a vendetta against Clary or the shadowhunters in general. They might use torture as a way to break her spirit and gain information about the shadowhunters' defenses.
Cell - like characters are often antagonists. Since Cell was such a major foe in the original series, new versions of Cell - like beings could be created in fanfiction to make Gohan angry. They might target his loved ones or the Earth itself, provoking Gohan's rage.
Even old enemies of the demigod world, who see Percy and Jason as threats, could be the antagonists. They might use deception and betrayal as a way to weaken the demigod forces, knowing that Percy and Jason are important figures among them.
The Akatsuki could be likely antagonists. They are always interested in powerful ninja like Naruto. They might use some forbidden jutsu to control Naruto to further their goals of collecting the tailed beasts. Another possibility is Orochimaru. He is always looking for ways to gain power and Naruto's power would be very appealing. He could try to control Naruto to use him in his experiments or to fight his enemies for him.
One likely antagonist could be a powerful new villain who has the ability to mind - control. This villain might target Deku specifically because of his potential as a hero. Another could be an old enemy of the hero society who sees Deku as a key to their revenge.
Obviously, Percy is one of the main characters as he is the one being tortured. The hunters as a group are also main characters. Among the hunters, there might be a leader or a few individuals who are more prominent in the act of torturing Percy. There could also be some of Percy's friends or allies who might try to rescue him, like Annabeth. They could be secondary characters who play important roles in the plot.
In many of these fanfictions, the demons are the main antagonists. They are the ones who carry out the torture on Sam. They might have different motives, like revenge for Sam's past actions against them or simply following Lucifer's orders.
The monsters native to Tartarus could also be antagonists. These could be huge, grotesque creatures that have never seen the outside world. They might attack Percy on sight, seeing him as prey. Some of them could be powerful enough to even give Percy a hard time, especially if his powers are weakened in Tartarus.
The other gods who oppose the relationship between Percy and the goddess could be antagonists. They might see it as a disruption of the natural order or a threat to their power. For example, if the goddess is from a certain pantheon and her pregnancy by a mortal like Percy goes against their rules, those traditionalist gods would be against it.