One possibility is that the female Kyuubi would try to calm him down. In the Naruto universe, the Kyuubi has a connection with Naruto. If it's a female Kyuubi in this fanfiction, she might use her influence to reach out to Naruto's inner self and bring him back to sanity. She could communicate with him in his mind, showing him memories of their bond and the people he cares about.
Well, he could start making really rash decisions. Maybe he'd attack his friends by mistake, thinking they were enemies. Since he's lost his mind, his usual judgment would be out the window.
In such a scenario, his relationships would be severely affected. His friends would be horrified by the change in him. Naruto's bond with Sasuke, for example, would be in jeopardy. Since he has lost his humanity, he may not understand the concept of friendship anymore and might even see Sasuke as a rival to be eliminated. And his relationship with the female Kyuubi would be complex. Maybe she would try to control him or use his new state for her own purposes.
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.
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.
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, Naruto could lose his way by being separated from his friends for a long time. Without their support and guidance, he may make wrong decisions. For example, he could be tricked into working for an enemy who disguises as a friend in need. This would lead him down a path where he gradually loses sight of his original goals and the positive ninja way he once followed.
Well, perhaps he loses his identity due to some sort of dark magic or a powerful enemy's jutsu. As a result, he could end up on a journey to rediscover himself. He might have to face many challenges without the confidence that his identity once gave him, like having to fight enemies without his well - known combat style.
The storyline might start with Naruto being put in a really difficult situation that causes him to lose his mind. Then, the female Kyuubi could be introduced as a key factor in his recovery. Maybe she has some special ability or knowledge that can help him get better.
One possible plot could be that Naruto's loss of sanity causes him to have a distorted view of the Fem Kyuubi. Maybe he sees her as both a threat and a source of power he can't control. This could lead to a conflict within himself as he battles his inner demons while also interacting with the Fem Kyuubi in unexpected ways. For example, instead of the usual relationship of cooperation or taming, he might try to completely dominate her, which would be a big shift from the normal Naruto we know.
The fem Kyuubi might feel a sense of protectiveness. Seeing Naruto without his mask could make her realize how vulnerable he is. She may then use her powers to safeguard him from any potential threats that could take advantage of his exposed state.
In the 'naruto finally loses it fanfiction', perhaps Naruto loses it due to continuous emotional trauma. He has been through so much, like losing his friends in battles or constantly being in life - threatening situations. All these accumulated stressors could cause him to snap. He could abandon his usual optimistic and friendly nature, becoming a more solitary and aggressive figure, which would be a huge shift from his normal self.