His sense of self - reliance could be greatly enhanced. In Tartarus, he can't always rely on his friends or his powers to get him out of trouble. This makes him more self - sufficient as he figures out ways to deal with the torture on his own. For example, he might learn new ways to defend himself against the strange and powerful foes he encounters there.
Percy may also develop a darker side to his character. The unrelenting torture can make him bitter and more vengeful. He starts to view the world from a more cynical perspective, but at the same time, this newfound edge also gives him an added determination to escape Tartarus and seek revenge on those who put him there, if there are any such antagonists in the fanfiction.
Percy might become more resilient. The constant torture in Tartarus forces him to dig deep within himself to find the strength to survive. He learns to endure pain and hardship in ways he never had to before, which toughens his mental state.
Percy might endure it through his own stubborn will. He's a demigod used to facing tough situations. In this fanfiction, he could be using his past experiences of fighting monsters and surviving hardships. He might tell himself that he's been through worse and this is just another trial he has to pass. So he grits his teeth and bears it as best he can, even when the torture in Tartarus is beyond what he's ever faced before.
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 becomes more responsible. Raising a child in Tartarus forces him to think about someone else's well - being all the time. He can't be as reckless as he used to be.
He might use his powers in creative ways to lessen the impact of the torture. For instance, if the torture involves some form of mental attack, he could try to use his connection to water to calm his mind. Also, his demigod abilities, like his superhuman strength and agility, could help him physically resist some of the more brutal forms of torture to a certain extent. And he may also draw strength from the memories of his past victories and the people he has saved.
It makes him more hardened. The harsh conditions force him to become tougher both physically and mentally. He has to constantly be on guard and ready to fight, which sharpens his reflexes and his ability to think quickly in dangerous situations.
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.
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.
Their relationship often deepens through shared hardships. In Tartarus, they rely on each other completely. For example, when facing a particularly tough monster, they have to trust each other's instincts and abilities, which brings them closer.
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.
Loki may become more cunning. His time with Thanos, a very powerful and dangerous being, forces him to be on his toes all the time. He might learn new ways to deceive and manipulate, which are skills he already has but are sharpened during this time.