There could also be various sea creatures that are personified and play significant roles. For example, a mermaid or a sea - horse - like creature that acts as Percy's guide through the complex underwater world of Atlantis. And don't forget the gods and goddesses who still have influence over Atlantis. Poseidon himself might make an appearance to guide or test Percy in this new setting.
Often, Percy's friends from Camp Half - Blood like Annabeth might also be involved. Annabeth could provide her wisdom and strategic thinking to help Percy navigate the political and magical challenges in Atlantis. Grover might also pop up in some way, perhaps using his connection with nature to communicate with the unique plant - like life forms in Atlantis.
Obviously, Percy Jackson is one of the main characters. He is the son of Poseidon and has his own set of powers and a strong sense of justice. Another important character could be the current prince or princess of Atlantis. They might be Percy's ally or rival, depending on the story.
The Titans could still be a major threat. Even though they were mostly defeated in the original series, some of their followers might see Percy's new role in Atlantis as a threat to their plans for chaos. They could try to undermine Percy's position in Atlantis and use the underwater city as a base for a new uprising.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
One interesting plot could be Percy exploring the hidden, ancient parts of Atlantis that are full of forgotten magic and powerful artifacts. Maybe he discovers a long - lost prophecy that only he can fulfill, which involves uniting different sea - based tribes in Atlantis.
The main villain could be a power - hungry Atlantean sorcerer who wants to use the power of Atlantis to rule both the underwater world and the surface. He might see Percy as a threat to his plans and try to eliminate him.
Grover can also be a great character in these fanfictions. His friendship with Percy is always a key element. When Percy becomes a prince, Grover might be the one to keep him grounded and remind him of his roots. He could also be involved in some comical situations as he tries to adjust to Percy's new royal lifestyle, like accidentally causing a scene at a grand ball or getting lost in a huge royal palace.
Obviously, Harry Potter would be one of the main characters. And there would probably be some Atlantian characters like the King or Queen of Atlantis, or other important Atlantian figures. Maybe some of Harry's friends from Hogwarts could also be involved, like Hermione and Ron, if they also get dragged into the Atlantis adventure.
Obviously, Atlas is a main character. He's often portrayed in different ways, sometimes as a more sympathetic figure trying to break free from his past, or as a more menacing antagonist still holding onto his old grudges. Percy Jackson also often shows up. His interactions with Atlas can range from confrontational to reluctantly allying with him.
Obviously, Percy Jackson himself is usually a main character. He is the one who gets involved in the devil - related situations. And then there could be new characters created specifically for the fanfiction, like a devil - like antagonist who is trying to corrupt Percy or the world around him.
The main characters are, of course, Percy Jackson and Frank. Percy is the son of Poseidon, a brave and resourceful demigod. Frank is a son of Mars in the Roman aspect, with his own set of unique powers and a shy yet determined personality. They often interact in these fanfictions, either as friends, allies in a quest, or sometimes having to overcome differences.
In this fanfiction, Percy Jackson is the key character. However, there could be new characters introduced who are related to the pets. Maybe there's a pet owner who Percy teams up with. Or perhaps there are veterinarians in the demigod world who help Percy when the pets get sick or injured. And the pets themselves, which could range from a cute little mouse with a big secret to a majestic eagle that can fly faster than the wind, would be important main characters in the story.