Percy's character could develop a more profound understanding of the fictional world. Since the Primordials are so ancient and central to the world's creation, their training would expose Percy to the deepest secrets. He might start to see things from a much broader perspective, not just in terms of his own personal battles but also in relation to the entire universe of the fanfiction. This new - found knowledge could make him more philosophical and less impulsive, as he now realizes the greater implications of his actions.
When Percy is trained by the Primordials in fanfiction, his character development can be quite complex. Firstly, he might gain a sense of detachment from his former self. The training is likely to be so intense and different from what he's used to that he may feel like a different person. He could become more calculating in his actions, as the Primordials' training might have instilled in him a need to think several steps ahead. Also, his relationships with other characters may change. Some may be in awe of his new - found power and knowledge, while others might be intimidated or even jealous. This new dynamic would further shape his character as he navigates through the fanfiction world.