There might be his friends too. They could be trying to help Percy get his mind back. So characters like Annabeth might be important as she would likely be deeply concerned about Percy's state and try to figure out what's wrong with him.
In this fanfic, it might be that Percy is in a high - pressure situation like a really difficult quest. Things keep going wrong, his team isn't listening to his plans, and he just loses his temper out of frustration. He starts yelling at his teammates, accusing them of not taking things seriously enough.
His own self - doubt could be the cause. Percy might be constantly thinking that he's not good enough to protect his friends or complete his quests. And one small trigger, like a friend making a harmless joke about his capabilities, could make him lose his temper because it hits too close to home.
A new, unknown villain could be the cause. This villain might have a special power related to mind control or mental manipulation. They target Naruto because of his status as a hero in the ninja world. By making Naruto lose his mind, they could throw the entire ninja world into chaos.
In this fanfiction, there could be a villain who has the ability to manipulate Naruto's mind. The villain might use this power to make Naruto turn against the Hidden Leaf Village. Naruto, without control of his own mind, could start causing chaos within the village, like attacking important buildings or trying to steal important ninja secrets.
One possible reason could be bullying. If Percy is constantly being picked on by other students, it might push him to lose his temper. Maybe they tease him about his appearance, his family, or his abilities. Another reason could be academic pressure. If he's struggling with his studies and feels like he can't keep up, it could lead to frustration and ultimately a loss of temper.
In this fanfiction, it could be that the pressure of being 'The Chosen One' finally gets to Harry. All the battles, the losses he has endured, and the constant threat of Voldemort might push him over the edge. He could start acting erratically, pushing away his friends and allies because he no longer trusts his own judgment.
Well, it could really damage his friendships. His friends might be shocked or scared by his outburst and start to distance themselves from him. They may not understand why he lost his temper and think he's become a different person. For example, if he yells at them during his fit of anger, they could take it personally and feel hurt.
Percy himself, of course. And his close friends like Annabeth would probably be there. She's always been by his side and would be likely to try and help him when he loses control. Grover might also be in it, as he's another important friend of Percy's. He could use his nature - related abilities to try and soothe Percy or help in some way.