Percy's personality would likely become more hardened. Growing up with assassins, he'd see a lot of violence and death. This would make him less trusting and more cynical. He might not be as quick to make friends as he was in the original story. However, his loyalty to those he does trust would be even stronger, like the loyalty assassins have to their comrades. Also, he'd be more disciplined, following the assassin code he was raised with.
His training would likely be intense. He'd probably learn stealth techniques like how to move silently and blend into the shadows. Swordsmanship would be a key part of it too, as he already has some skills with a sword. And he'd be trained in hand - to - hand combat, learning different fighting styles. Maybe he'd also study poisons and antidotes, just in case he ever needed to use or counter them during his assassin - like missions.
His character would become more confident in his artistic abilities. Apollo is the god of many arts, so Percy would likely be exposed to music, poetry, and painting. He might be more flamboyant in his actions and speech, as Apollo has a certain charm. For example, he could start using more flowery language when talking to his friends or enemies.
Percy's character would likely become more in tune with nature. He'd be more reserved perhaps, as he grows up in an all - female group. His sense of honor might be more shaped by Artemis' values, such as protecting the weak and the natural world.
In these fanfictions, Percy's character is often more edgy. He has to be more calculating and less impulsive compared to his original self. Since he's dealing with the assassin - like role, he can't just rush into things. For example, he might take more time to plan his attacks against enemies.
One idea could be that Percy is framed as an assassin within the demigod world. The other demigods start to hunt him down, but he has to prove his innocence while also uncovering the real assassin's plot. Another could be an alliance between Percy and some assassin - type characters from other mythologies to take on a common enemy that threatens both their worlds. Maybe a powerful titan or a new, dark god.
Percy himself would be very interesting. His powers and his sense of justice could create a lot of conflict if he were involved with assassins. Another interesting character could be Annabeth. Her strategic mind would make her either a great opponent or an ally to assassins. Also, Nico di Angelo with his connection to the underworld could bring a dark and mysterious element to the story as an assassin or someone related to them.
The plot could be centered around Percy's identity crisis. Since he's raised by Apollo, he might struggle with whether he should follow Apollo's more flamboyant and carefree ways or stay true to his own nature. He could be exposed to Apollo's many lovers and children, which would make him question his place in this complex family. Additionally, his relationship with other demigods would change. He might be seen as an outcast by some who are loyal to Poseidon, while others might be jealous of his connection to Apollo. This would lead to various adventures where he has to prove himself not just as a son of Poseidon but as someone with a unique upbringing under Apollo.
If Percy Jackson was raised in the Underworld, his personality would likely be much darker. The environment of the Underworld is gloomy and full of death. He could be more brooding and less trusting. His fighting style might also change, incorporating more of the Underworld's magic and powers. He may even have a different weapon, something more associated with the Underworld like a bone - made sword or a whip made from the shadows of the dead.
One possible plot could be that Percy, being raised by Artemis, grows up with a deep connection to nature and the moon. He might be trained in archery and hunting from a young age. Since Artemis is a virgin goddess, Percy would have to deal with the unique situation of being part of her all - female group, perhaps facing some initial resistance from the hunters. He could also develop special powers related to the moon or gain an enhanced sense of stealth.