For his friends, it could be a huge blow. They might feel abandoned and worried about his safety. For example, Annabeth could be extremely distraught, constantly worried about Percy's well - being. Some of the younger campers who looked up to Percy might lose their sense of hope and inspiration. They may feel less secure without him around.
The camp leaders would likely be concerned. Chiron might try to send out search parties or try to figure out where Percy went and why. Some of the other powerful demigods at the camp might see this as an opportunity to step up, but they could also be resentful if they feel Percy left without proper notice or for selfish reasons. It could also create a rift in the camp's unity as different characters have different opinions on Percy's departure.
His friends might feel sad and abandoned. For example, Hermione and Ron could be heartbroken at first, missing their best friend and wondering if they could have done something to make him stay.
For Tony Stark, it could be a blow to his ego. He might see Peter's departure as a sign of failure in his mentorship. He could also start to question his own actions and decisions that led to Peter leaving.
Arya might be inspired to go after him. In fanfiction, she could see his departure as a call to adventure herself. She might feel that there are things out there that she can learn from Jon or things she can do to help him. His leaving could be a catalyst for her own journey outside of Winterfell.
For Hermione, it could be a huge loss. She might feel abandoned and try to find ways to bring him back. Ron, on the other hand, might be angry at first but then also miss his best friend terribly.
His friends, like Annabeth, would be deeply affected. She would probably be heartbroken and confused at first. But she might also be one of the first to try and understand Percy's motives and see if there's a way to fix the situation or bring him back to his senses. And for the younger demigods, Percy was like a hero, so his actions could shatter their beliefs and make them question everything they thought they knew about the world of demigods.
The town of Beacon Hills itself might be affected. Stiles was always in the middle of things, helping to solve supernatural mysteries. Without him, there could be an increase in unexplained events. And some of the townspeople who knew Stiles, like Sheriff Stilinski, would be deeply affected on a personal level, with the sheriff possibly becoming more overprotective of the remaining youth in town.
For Sakura, it could be a huge emotional blow. She might feel abandoned and be motivated to become stronger herself to bring them back. Also, she might have to step up in their absence and take on more responsibilities in the village.
The other Jedi would be affected in different ways. Some might see her as a traitor and become more strict in their views, while others might secretly sympathize with her. It could create a divide within the Order in the fanfiction.
Sakura would be deeply affected. She has been Naruto's friend for a long time. His departure might make her feel sad and also a bit lost. She might worry about his safety. Sasuke too would be affected. He and Naruto have a complex relationship. Naruto's leaving could make Sasuke re - evaluate his own place in Konoha.
Hermione would likely be very curious about the reasons behind Snape's departure. She might try to research it in the library, looking for any clues or hidden motives. She could also be worried about the impact on the overall balance of Hogwarts, as Snape was an important teacher, especially in Potions.