Grover could be there too. He might be like a little helper or friend to baby Percy. Grover could be curious about Percy's connection to Poseidon and try to befriend him from a young age. He might also be the one to bring news from the surface world to Poseidon and Percy, like what's going on in the human world among the demigods.
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.
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.
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.
Maybe it's the child of one of his friends or allies. For example, it could be the baby of Ron and Hermione. Harry would then be taking care of the baby out of loyalty and friendship.
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.
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.
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.
Well, clearly Harry is one of the main characters as he is the one taking care of the baby cousin. The baby cousin is also a main character, even if it's a young and not fully developed character yet.
Annabeth could be there. Since she is Percy's friend, she might come in to help Percy when he's in trouble during his growth with Poseidon. Also, other demigods from the camp could show up. They might be curious about Percy's special relationship with Poseidon and how it's affecting his growth.
In fanfictions, Poseidon often interacts with baby Percy in a very gentle and protective way. He might use his powers to create a safe environment for Percy, like a special underwater grotto where Percy can be hidden from danger. Sometimes, he sends sea creatures to watch over Percy, like dolphins or friendly sea nymphs.
Annabeth is often a popular character. She could be in a story where she has to help Percy deal with his relationship with Poseidon. Maybe she gives him advice on how to handle Poseidon's overbearing nature or how to make the best use of the gifts Poseidon gives him.