His character could change in several ways. He would have a new sense of identity. No longer just a demigod with powers, but now something more 'aquatic'. He might be more curious about his father's world and want to spend more time there. This could lead to some conflict with his human side, as he has to balance his life on land and his new underwater existence. Also, he could become more of a leader among the sea creatures, as he has a unique connection to them through the tail.
It might make Percy more confident. With the trident, he has a powerful symbol of his father's support. He could use it to face stronger enemies and gain more respect in the demigod world.
Well, perhaps the plot could involve Percy being given the tail as a sign of special favor from Poseidon. He might then have to learn how to use the tail in underwater adventures, like swimming faster or having better control in the ocean currents. It could also lead to him being able to communicate more effectively with sea creatures, since he now has a more 'aquatic' form.
The dynamic between Poseidon and his son, Percy (in the original), would be completely flipped. Instead of a father - son relationship, they would be more like two separate entities with Poseidon now in Percy's role. Poseidon would have to face the same challenges Percy did, but without the advantage of Percy's unique experiences as a demigod growing up.
He'd probably be more confident in his water - related abilities from an early age. Since he's raised by Poseidon, he might have a deeper understanding of the sea and its power.
It can also make Percy more empathetic. Since he has such a powerful and protective father, he realizes the importance of protecting others too. He may be more likely to step in and help those in need, just as Poseidon does for him. This protection also gives Percy a sense of security that allows him to explore his own identity and powers more freely. He doesn't have to constantly worry about being alone or unprotected, so he can focus on growing as a hero and as a person.
In this fanfiction scenario, Poseidon's relationship with Percy would be full of twists and turns. At first, Poseidon might be shocked and a bit angry. How could his son be related to Leto? But as time goes on, he realizes that Percy is still his son and that this new connection could be used to their advantage. He starts to work with Percy more closely, guiding him on how to balance his powers from both Poseidon and Leto. They might have some father - son talks about the new responsibilities Percy has and how he can use his dual heritage to protect the world better. Poseidon could also start to see Percy as more of an equal, as he is now part of a more complex and powerful family tree.
Fem Percy in this fanfiction would likely start as a somewhat timid character, given the enormity of having Poseidon as her father. But as she grows, she'd gain confidence. Poseidon's influence would make her more attuned to the power of water, and she'd learn to use it in creative ways. Her experiences with Poseidon would shape her into a more complex character. She might be more diplomatic, using her charm and intelligence to solve problems. Also, she could develop a fierce loyalty to those she cares about, and her growth would be marked by her ability to balance her demigod powers with her emerging femininity, standing out as a unique figure in the world of Greek mythology - inspired fanfiction.
Poseidon's comfort can make Percy more confident. For instance, if Poseidon tells Percy he has great potential, Percy will start to believe in himself more. This new - found confidence can lead to him taking on more challenges bravely.
Well, it would make Percy's relationship with Poseidon extremely complicated. Since Amphitrite is Poseidon's wife in Greek mythology, if she's Percy's mother, it means Poseidon has a different kind of connection to Percy. Percy might feel more conflicted about his fatherly figure. He could be in a strange position where he has to navigate between the relationship with his mother and the traditional view of Poseidon as a powerful and somewhat distant god - father figure.
Their relationship could become more intense. The hickeys might be a sign that they are moving to a more physical and passionate stage. Percy might become more protective of Annabeth as well, seeing how she's so openly showing her affection in this way.