If Percy was born in Tartarus, his character development would be a long and arduous journey. He would start with a basic instinct for survival. As he grows, he would constantly struggle with his identity. Is he a monster because he was born in Tartarus? Or can he be a hero? His moral compass would be constantly tested. He would encounter situations where the line between good and evil is blurred. For instance, he might have to make deals with some of the less savory creatures in Tartarus to survive. All these experiences would shape him into a complex and unique character, very different from the Percy we know in the original series.
Percy's development would be centered around survival and power. In Tartarus, it's all about being strong enough to endure. So he would be more focused on getting stronger. He might also be more isolated, as not many would understand his Tartarus - born nature. His relationships would be more difficult to form and maintain. He would have to learn to control his powers, which might be more closely related to the dark forces of Tartarus, without being consumed by them.
Percy's character would likely become more hardened. Living in Tartarus, he'd face constant danger and that would make him more cautious and strategic. His sense of survival would be enhanced.
In fanfiction, if Percy is sent to Tartarus, his character development could be complex. He would likely become more self - reliant as he can't always count on his friends to save him. His view of good and evil might also change. He may realize that there are different shades of evil in Tartarus, not just the simple 'bad guys' he faced before. This could make him more empathetic in some ways, but also more cynical in others. He'd also grow in terms of his strategic thinking, as every move in Tartarus could mean life or death.
Percy could become more self - reliant. In Tartarus, he can't always count on his friends or his powers in the same way as on the surface. So he'll have to dig deep within himself to find new strengths. He may also become more strategic in his thinking, as one wrong move in Tartarus could mean death.
His character could develop in terms of his understanding of the divine world. Since he's with a goddess, he'd get a closer look at how the gods operate, their politics, and their values. This would make him more aware of his place in the grand scheme of things, not just as a demigod but as someone with a connection to a powerful goddess. He might also become more diplomatic as he tries to navigate the complex relationships within the divine realm.
He would likely be more cautious. Since he's born with this connection to monsters, he might be more aware of the dangers around him from a young age. He'd be less likely to rush into things like the normal Percy does. For instance, when he first hears about Camp Half - Blood, instead of just going along with Grover, he might be more hesitant and ask a lot more questions because he knows the potential threats monsters pose.
Percy's character would be much more of a warrior. In Sparta, they were bred for war. Percy would have a different mindset. He'd be more aggressive in combat and less hesitant to use force. His sense of honor would be deeply ingrained in the Spartan way. He'd also be more used to a hierarchical society, so his interactions with others might be more formal. For example, he'd show more respect to his superiors based on their rank. His relationship with the gods would also be different. He'd have to hide his connection to Poseidon more carefully as the Spartans had their own pantheon of gods they revered, and any deviation from that could be seen as heresy.
His water - based powers might intensify as he is in a place filled with so much negative energy. He could learn to control the strange, dark waters in Tartarus, which are probably more powerful and unpredictable than normal water. Maybe he can use them as a shield or a weapon in ways he never thought of before.
Percy's view of the mortal world would expand. Being in the army, he'd see a different side of mortals - their teamwork, their sense of duty. He might also learn new skills like military strategy that could enhance his demigod fighting abilities. His loyalty might be tested as he has to balance his loyalty to his demigod friends and his new military comrades. And his sense of self - worth could change as he discovers new ways to contribute in a military setting.
On the other hand, Percy could become more of a revolutionary figure. His act of killing Zeus could inspire other demigods to stand up against the unjust ways of the gods, and Percy would have to step into a leadership role, which would force him to grow in terms of strategy and diplomacy.
If Percy were a speedster in fanfiction, his confidence would likely grow. He'd be able to get out of tough situations more easily, which would make him bolder in his actions.