Monsters from the Underworld could also be antagonists. They might not like that Percy has Hades' favor and try to attack him to prove themselves or stop him from using the blessing for good.
The other gods who are jealous of Percy's new connection with Hades could be antagonists. For example, Zeus might see Percy as a threat now that he has Hades' blessing.
Other jealous demigods could act as antagonists. They might feel that Percy doesn't deserve the blessings and try to undermine him. They could spread rumors or even try to sabotage his quests to make him look bad in the eyes of the gods.
Well, perhaps the plot could involve Percy being given some special powers or protection by Hades. Maybe he's in a dangerous situation and Hades' blessing helps him survive or gain an advantage over his enemies.
Some of the more malevolent spirits or monsters from the Underworld could also be antagonists. They might resent Percy for being Hades' adopted son and try to undermine him. For example, a powerful undead general who believes he should be the one favored by Hades instead of Percy.
Another option could be some rogue demigods who are jealous of the son of Hades' powers. They might form a group and try to cause trouble for him and Percy Jackson. They could be working with some minor gods who have a grudge against Hades, creating a complex web of enemies.
Some of the more powerful and vengeful spirits in the Underworld itself could be antagonists. They might not want an outsider like Percy to be the Champion of Hades. They could try to undermine him at every turn, like spreading false rumors about him among the other Underworld denizens or directly attacking him when he's vulnerable during his quests in the Underworld.
The other gods who don't trust Hades might be antagonists. They could see Bella as a threat because of her relation to Hades. For example, Zeus might be worried that Bella could one day try to overthrow him or cause trouble in the order of the gods.
The Akatsuki could still be the main antagonists. They might see Naruto's new connection to Hades as a threat or a powerful tool they can use. They would try to capture him to either control his powers or eliminate him.
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.
Since the title mentions Percy, he's most likely one of the main characters. But without reading it, it's hard to say who else might be important. There could be characters from the original series that Percy is from, if this is a fanfiction based on an existing franchise. Maybe his friends or mentors could be involved too.
In non - lemon fanfiction, Percy is often depicted as brave and resourceful as in the original series. His loyalty to his friends and his struggle with his identity as a demigod are common themes. Hades, on the other hand, is usually portrayed as a complex figure. He is powerful and mysterious, with his own motives and a sense of duty towards the underworld. Their relationship might be explored in terms of mutual respect or a begrudging alliance.