His story changes a great deal. He can't rely on his powers to solve problems. For instance, in battles, he'd have to use normal weapons or his intellect. His relationship with Annabeth would also be different. They might not have the same kind of adventures together. He'd be more in danger from the mythical world since he's no longer a powerful demigod.
When Percy becomes mortal in fanfic, his story takes a new turn. Firstly, he has to adapt to a normal lifespan, which is a big change from his potentially long demigod life. His connection to the gods becomes more tenuous. He may not be able to get their help as easily. Also, his place in the demigod community at Camp Half - Blood would be altered. He might feel like an outsider, and his friends may treat him differently. He'd have to find new ways to contribute and be useful. This could lead to him exploring other aspects of his personality, like his creativity or his ability to lead in non - combat situations.
His social circle would expand. He'd meet new people in the music industry, from producers to fellow singers. His daily routine would shift from demigod training to rehearsals and recording sessions.
His personality might become more reserved. As a Cullen, he'd have to fit into their more subdued lifestyle. He might lose some of his brashness.
His character would become more detached. Percy is usually very involved and emotional in his relationships, but as Death, he would need to be more aloof. He can't be too attached to the living as his new role demands a certain level of impartiality towards life and death.
He might become more authoritative. As the Titan King, Percy would have to command respect and make tough decisions. He can't be as carefree as he was before.
His confidence would likely soar. As a god, he'd know he has more power and capabilities. So he might be more assertive in his actions and decisions.
In this fanfiction scenario, Percy's character change would be quite drastic. He would probably start to embrace the more chaotic and power - centric nature of the Titans. His relationships would be in turmoil. For example, his relationship with Annabeth would be severely tested. He might not value the same things he once did, like friendship and the protection of the camp. His new Titan - influenced self could be more self - serving, seeking to gain as much power as possible and not caring about the consequences for those around him. His values would be realigned with the Titans' more cut - throat approach to power and survival.
If Percy becomes influenced by Kronos in this fanfiction, his sense of morality would be greatly affected. He would probably lose his sense of right and wrong which was a big part of his character before. His loyalty towards Camp Half - Blood and his friends would be in jeopardy. He might start to think that the ends justify the means, which is a very Kronos - like way of thinking. And he could become more aggressive and less patient, as Kronos was always in a hurry to regain his power and rule over the world.
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.
He might become more solemn. As a god, he has more responsibilities. So, he can't be as carefree as he was as a demigod. He would need to think about the big picture all the time.
His character might become more aloof. As a primordial god, he would be far removed from the normal world of demigods. He'd likely have a different perspective on things like friendship and loyalty. For example, he might not be as quick to jump into a fight for his friends as he used to be, because his new status would bring a different set of responsibilities.