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.
Another possible main character could be a new antagonist. Maybe there's someone or something that caused Percy to lose his mind in the first place, and this entity could play a major role in the fanfic as it continues to oppose Percy's friends' attempts to restore his sanity.
Well, perhaps Percy starts seeing things that aren't there. Maybe he gets really paranoid and starts suspecting his friends of things they didn't do.
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.
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.
It could be an old enemy who takes advantage of Percy's weakened state. For example, Kronos might see this as an opportunity to finally defeat Percy and take over Olympus. Since Percy has lost an arm, Kronos might think he's more vulnerable and launch a new attack.
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.
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.
I'm not sure specifically as it depends on the fanfic. But it could be characters who are masters of the mind arts, like a wise old mentor who teaches others about it. Or perhaps a young and talented individual who is just starting to explore their mind arts capabilities.
It could be a power - hungry warlord in the story's setting. This warlord might be the one who ordered an attack on Carl's group, leading to Carl losing his eye.
Perhaps an unknown and powerful dark ninja. This ninja could be from a long - forgotten clan with dark powers. They might have a vendetta against the Leaf Village and see Naruto as the key to their revenge. So they use their dark arts to make Naruto lose his sanity and turn him against his own village.