Percy would likely become more cold - hearted. His loyalty to Voldemort would make him turn a blind eye to the harm being done to others. He might also become more power - hungry as he is influenced by Voldemort's thirst for domination.
Percy's character would be warped. At first, he might seem more confident as he is associated with a powerful figure like Voldemort. But as time passes, he would become morally corrupt. His values would shift from the ones he was taught growing up. He would no longer care about things like fairness and equality. Instead, he would focus on pleasing Voldemort and maintaining his position in the Dark Lord's inner circle. This would lead to a complete transformation of his personality, making him unrecognizable to those who knew him before.
His character would change drastically. He could start to isolate himself from his old friends and family. He might become more secretive and calculating. For example, he would have to hide his friendship with Voldemort from those who oppose the Dark Lord, which would make him constantly on edge and more distrustful of others. He may also start to adopt Voldemort's cruel methods of dealing with those who oppose him.
Percy would probably become more in tune with emotions. He would be able to sense the feelings of those around him more acutely. This could make him more empathetic but also more vulnerable. His relationships would change significantly. For example, his relationship with his mother, Sally, might take on a new dimension as he discovers more about his divine heritage related to love. And his relationship with Annabeth would be tested as he has to balance his newfound charm and emotional awareness with their existing bond.
His relationship with his friends might change. Some of his friends who are more used to the 'old Percy' might be a bit shocked or even worried about his new weapon choice. On the other hand, he might attract new allies who are more into modern - day combat methods. Overall, his character would have to adapt to the new implications and responsibilities that come with using a gun.
He'd probably be more aware of his appearance. Instead of just being a scruffy, normal - looking demigod, he might take more care in how he presents himself. He could also be more charming without really trying, which might get him into trouble sometimes as people might misinterpret his intentions.
His personality would probably be more cold and calculating. Voldemort's influence would make him less empathetic. He might be more ruthless in achieving his goals.
Well, it would completely mess up the existing relationships in the original story. For example, the Weasley family's connection to the wizarding world's power dynamics would be drastically different. Percy, who was always a bit of a stickler for the rules in the original, might have some internal conflict knowing his family has such a dark connection. Also, it could lead to some really interesting interactions between Percy and the Death Eaters or other Voldemort supporters. Maybe Percy would be targeted more by both sides, the good because they'd be worried about his loyalty and the bad because they'd want to use him for Voldemort's purposes.
His character would likely become more hardened. The military life is tough and Percy would have to face a lot of difficult situations. He might become less trusting at first as he'd be in a new environment full of strangers. But over time, he could learn to rely on his new comrades. Also, his sense of responsibility would expand from just protecting his friends and family to protecting his entire unit and his country in the context of the fanfiction.
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.
He might start to target the authors of the fanfiction if he didn't like how they portrayed him.
Harry would likely become more cunning. He has to survive in Voldemort's lair, so he'd learn to hide his true feelings and intentions. For example, he might pretend to be more submissive than he really is to gain Voldemort's trust slightly.
He might become more vulnerable. Usually seen as a powerful and cold - hearted villain, losing his lover could expose a softer side. He could start to show emotions like sadness or regret that he has never shown before.