He'd lose his sense of justice. Percy was always about protecting the weak and fighting for what's right. As an evil character, he'd probably start oppressing the weak instead. His once - kind heart would turn cold.
Percy's relationships would completely change. He'd turn against his friends like Annabeth. Instead of their loving bond, it would become one of hatred. His loyalty, which was a big part of his character, would be directed towards the wrong things, like serving evil forces. He might also become more self - centered, only caring about his own power and dominance rather than the well - being of others.
If Percy goes evil in fanfiction, his whole moral compass would flip. His actions would no longer be guided by the values he once held. He would be more likely to deceive and betray. His attitude towards the gods would be different too. Instead of respecting or at least tolerating them, he could actively seek to undermine them. His leadership skills, which were used for good before, could be used to lead a group of villains, causing destruction on a large scale.
He would become more cynical. His once - positive view of the world and people would be replaced with suspicion and hatred. He might start to see everyone as an enemy.
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 friends could start by trying to figure out what caused the transformation. Annabeth might use her wisdom to research in the library at Camp Half - Blood. Grover could use his connection to nature to see if there are any magical plants or creatures that could reverse it. And the other demigods could keep Percy safe from any potential threats while he's in his cat form.
Percy's diet would be a major change. Instead of his normal diet at Camp Half - Blood, he would have to feed on blood. This could lead to some really interesting moral dilemmas in the fanfiction. Does he feed on humans? If not, how does he get the blood he needs? Maybe he tries to find a way to survive on animal blood, like some vampires in other stories do.
In his family relationships, Sally Jackson would be in for a big surprise. She would have to adjust to seeing her son as a daughter. Percy would have to deal with different family expectations as a girl. His relationship with Poseidon would also be different. Poseidon might be confused and perhaps less likely to immediately offer the same kind of support he did when Percy was a boy. And Percy's romantic interests, if any, would have to be re - evaluated. He might find himself attracted to different people or be the object of different types of attraction.
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.
In a 'Percy Jackson turns evil' fanfiction, he might use his powers against his former friends. For example, his control over water could be used to flood Camp Half - Blood. His knowledge of the demigod world would also be a weapon in his hands to outwit and defeat those who oppose him.
His confidence could turn into overbearing arrogance. Instead of being a source of motivation, it would be a tool to belittle others. His determination might become a stubborn and unyielding obsession with power and domination.
His personality would likely become more protective. Since Cerberus is a guardian, Percy might feel an intense need to protect those around him more than ever.