It could be that Percy's sense of honor led him to sacrifice himself for Zoe. If he had made a promise to protect her, he would uphold it no matter what. Also, Zoe might have been the key to solving a major mystery or threat. Percy, realizing this, chose to sacrifice himself so that Zoe could continue and complete whatever task was necessary. Additionally, his love for his friends and the greater community of demigods might have made him believe that Zoe's survival was more important than his own.
One reason could be to create drama. Writers often like to shake things up in the relationships of well - known characters. Another might be to explore different sides of Zoe's character, perhaps showing her as more complex or flawed than she appears in the original work.
It might also be related to some sort of enemy or threat that specifically targets Percy and his relationship with Annabeth. The enemy could be so powerful that Percy realizes the only way to stop it from harming Annabeth is to sacrifice himself, like if the enemy feeds on his life force or his connection to Annabeth, and he chooses to end it by sacrificing his own life.
One possible reason could be her deep love for Percy. She might see his life as more valuable in the grand scheme of things, perhaps because of a prophecy or a mission that only he can complete. So, she chooses to sacrifice herself to ensure his survival.
Percy could be a brave and adventurous character, perhaps with some special abilities like being able to control water or having enhanced strength. Zoe might be a smart and resourceful female character, maybe with skills in tracking or archery. They could be from a fictional world full of magic and mythical creatures.
Well, his sacrifice would likely have a huge impact. For Zoe, she might be filled with guilt and a new sense of purpose. The other characters like Annabeth could be deeply affected too. Annabeth might be angry at first that Percy made such a choice, but then she could use that anger to fuel her own actions in the story. His sacrifice could also inspire the other demigods to be braver and more selfless in their own quests.
Maybe to save the Justice League from a powerful enemy. He might sacrifice himself if it means the others can escape or defeat the foe.
It could be because Percy is different. If he comes from a different background or has a unique personality trait, the bullies might target him. In the fanfiction, if Percy is from a different race or has an unusual way of speaking, the bullies might pick on him for not being like them.
Maybe he loves her deeply. In the Pokémon world, love can be a strong motivator. He might see a threat coming towards Serena and know that sacrificing himself is the only way to save her.
Perhaps it's part of a grand plan that Poseidon has, one that he believes will ultimately benefit Percy but in the short - term causes him pain. For example, he might be trying to toughen Percy up or make him prove himself in a more extreme way, thinking it will help Percy in the long run.
It could be due to physical injury. Maybe he got hurt in a battle or an accident within the fictional world of the story.