Well, he might become more ruthless. He could start making decisions without the normal moral compass that his humanity provides. For example, he might be more willing to use extreme violence to achieve his goals, not caring about the harm it causes to others.
In this kind of fanfiction, the plot might involve Bella becoming a more ruthless and self - centered character. She may no longer be bound by her usual moral and emotional constraints. For example, she could start to disregard the feelings of her friends and family. She might also engage in more dangerous or unpredictable behavior, like getting involved with the wrong crowd in the vampire or werewolf world.
First off, Edward is likely to be deeply affected. He has spent so much time and energy trying to protect Bella and keep her safe. Now, with her humanity off, he may feel like he has failed. Jacob, too, will be affected. His feelings for Bella, whether it's love or friendship, will be challenged. Also, Bella's father would be affected. He has always cared for his daughter, and seeing her change so drastically would be heart - breaking for him.
One possible reason could be to protect himself from the pain and loss he has endured. After all he's been through, like the death of his parents, Sirius, and so many others, turning off his emotions might seem like a way to avoid further hurt.
His powers might not be affected at all. Percy's connection to water is more about his essence as a demigod rather than his physical form. So he could still control water, summon sea creatures, and heal himself with water just as well as before.
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.
Well, his confidence might shift. As a woman, Percy could be more cautious at first, having to deal with different social expectations. But he could also become more empathetic. Since society often expects women to be more in tune with emotions, Percy might start to pick up on the feelings of others more easily.
His relationship with Annabeth would be different. She might have to get used to Percy being a pegasus. There could be some initial awkwardness, but also a deeper understanding as she sees him in a new form.
It could be a story where Percy takes his shirt off during a training session. Maybe he's getting ready for a really intense sparring match and removing his shirt is part of his pre - fight ritual to show his determination and focus.