Well, having wings could make Percy more independent. He can now reach places on his own without relying on his friends as much. This might cause some tension in his relationships at first, but it also forces him to grow and learn how to balance his new independence with his need for his friends. For example, he might fly off to scout ahead during a quest and have to make quick decisions on his own, which is a big step in his character development.
Perhaps it's a gift from his father, Poseidon. Since Poseidon has control over the sea and all things related to water, he might have some connection to a power that gives Percy wings, like as a way to help Percy reach new heights both literally and metaphorically.
In many such fanfictions, Castiel's character development often revolves around his newfound vulnerability. With his hurt wings, he can't be as strong or as mobile as before. This forces him to rely on others more, which makes him more empathetic and less of the aloof angel he used to be.
One interesting story could be about Percy using his wings to reach new and hidden places in the demigod world. Maybe he discovers a secret island where ancient monsters lurk. With his wings, he has an advantage in exploring and fighting them off.
The black wings could give him an extra means of mobility. He could use them to quickly change direction during combat, swoop down on opponents from above, or make quick getaways.
If Percy Jackson was raised by a warrior in fanfiction, his character would likely become more battle - hardened from an early age. He might be more confident in his combat skills and less hesitant in dangerous situations. For example, instead of gradually learning to fight monsters as in the original story, he could already have a warrior's instincts, making him a more formidable opponent right from the start.
The wings could be used in combination with his Pokemon's attacks. For example, he could use his wings to create updrafts for Flying - type Pokemon to enhance their attacks. Or he could distract opponents while his Pokemon sneaks up from behind. Also, the psychological impact on his opponents would be significant. Seeing Ash with wings would be unexpected and might throw them off their game, giving Ash an edge in battles.
If Naruto has wings, his hair might seem to flow more gracefully in the wind as he flies. His eyes could glow a different color when he uses the power associated with the wings. Also, the wings themselves could be unique. They might be made of some kind of chakra - like substance, giving them a shiny or translucent look. And there could be markings on the wings that are related to his Uzumaki heritage or some new power he has acquired.
His overall posture and movement would change too. With the wings, he would need to be more aware of his surroundings to avoid knocking things over with them. His gait might be different as he adjusts to the new weight and balance on his back. Also, his face might have a more 'otherworldly' look. Maybe his eyes would have a glint like a dragon's, or his skin could take on a slightly scaly texture near the base of the wings.
Well, having raven wings might also change his chakra usage. He could potentially use the wind nature of his chakra in new ways. For example, he could create stronger gusts while flying to disrupt enemy formations. Also, the wings could be used defensively, like shielding himself or his comrades from attacks.
One possibility in this fanfiction is that the wings change Percy in some way. They might give him a new perspective, literally and figuratively. As he soars through the skies, he could see things he's never noticed before on the ground. This could lead to him discovering new secrets about the demigod world or even his own powers. He might also face new challenges, like dealing with air - based monsters or learning to fly in different weather conditions.