Chiron, the centaur who trains demigods at Camp Half - Blood, could also play a role. He might offer advice to Percy before he embarks on his journey with Poseidon or be there to welcome him back and help him adjust to life at camp again after his time with his father. His wisdom and experience can be a great addition to the story, especially in terms of Percy's growth as a demigod.
Athena might be involved. She could be a bit wary of Poseidon taking care of Percy, as she has her own ideas about the future of heroes. She might occasionally check in on Percy, just to make sure Poseidon isn't influencing him too much in a way that goes against her plans.
One interesting plotline could be Poseidon teaching Percy some ancient and powerful water - based magic that is known only to the gods. This could give Percy an edge in his battles.
Annabeth would definitely be involved. As Percy's girlfriend, she is often by his side during his adventures. She could be the one who witnesses Poseidon's pride towards Percy and helps Percy understand the significance of it. Grover, Percy's best friend, is also likely to be there. He might provide some comical relief during the serious moments when Poseidon is showing his pride. Also, other demigods from Camp Half - Blood like Jason, Piper, and Leo could be involved. They could be in awe of Percy's new - found status with Poseidon and might interact with him differently because of it.
Obviously, Percy Jackson and Poseidon are the main characters. Percy, as the demigod son of Poseidon, has his own set of powers and a complex personality. He's brave and often impulsive. Poseidon, on the other hand, is a powerful god of the sea with his own motives and a sense of fatherly love towards Percy, which is often explored in these fanfictions.
Luke might also make an appearance in some fanfictions, even though he's dead in the original series. His past grievances against the Olympians could be a driving force for Percy's new opposition, or he could be a ghostly advisor to Percy, guiding him in his fight.
Obviously, Percy and Calypso are the main characters. Percy, with his heroic deeds and complex past, and Calypso, the lonely figure on her island. Sometimes, other characters from the Percy Jackson universe like Annabeth might also be involved, perhaps she tries to convince Percy to come back but fails as he has made up his mind to be with Calypso.
One interesting scenario could be Poseidon using his powers to create a mini - underwater world for baby Percy to explore safely. He might make colorful fish swim around Percy and gentle sea currents act like a cradle.
The gods themselves, like Zeus, Poseidon, and Athena are also main characters. Their different personalities and powers play a big role in the fanfiction. For example, Zeus's authoritative nature might create conflicts, while Poseidon's relationship with Percy as his father adds an extra layer of complexity.
The main characters are usually Percy Jackson himself, as he is the central figure in the series. Annabeth Chase is also often involved, given her close relationship with Percy. Grover Underwood is another character that commonly shows up in these fanfictions, as he is Percy's best friend.
Well, clearly Percy and Poseidon are the main characters. Percy, as a baby, is full of potential and mystery. And Poseidon, being his father, has a big role to play in protecting and guiding Percy.