In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.
Madara Uchiha could also be a likely cause. If he somehow managed to return or had a long - planned scheme that involved breaking Naruto's sanity. He was a very powerful and devious foe who could use his knowledge of the ninja world and its powers to target Naruto's mental stability.
You need to focus on showing his gradual descent into madness through his thoughts, actions, and interactions. Build up the tension and show the triggers that push him over the edge.
In fanfic, Yumeko is often depicted as bold and fearless. She's not afraid to take risks, especially when it comes to gambling. Mary, on the other hand, is usually portrayed as more calculating and strategic. She thinks things through carefully.
Another is the discovery of a dark secret about his family or the village. Maybe he finds out that the village leaders had some malicious intentions towards his family all along. This realization could be too much for him to handle and make him lose his sanity.
Iruka would be a likely candidate. He was Naruto's first teacher and has always cared about him. He would probably notice the signs early on and try his best to bring Naruto back from the brink of insanity. He might use his kind words and the bond they share to reach Naruto.
In Kakegurui fanfiction, Ryota and Yumeko's relationship is a fascinating subject. Yumeko's insatiable appetite for gambling and her unique charm make her a central figure. Ryota, on the other hand, is often in a position of trying to understand her. Their interactions are filled with tension and drama. The fanfiction can play with their differences in personality. For example, Yumeko's impulsiveness against Ryota's more reserved nature can lead to some really engaging storylines. It could be that Ryota is gradually influenced by Yumeko's way of life, or he tries to change her for the better. There are so many possibilities in exploring their relationship through fanfiction.
Common themes in their fanfiction include the idea of redemption. Maybe Ryota is trying to redeem Yumeko from her extreme gambling ways, or vice versa. Another theme is the battle between logic and instinct. Ryota represents logic most of the time, while Yumeko is all about following her instincts in gambling. And there is also the theme of loyalty. How far will Ryota go to be loyal to Yumeko, and how will she respond to his loyalty? These are all things that are often explored in fanfiction about them.
A common theme is rivalry. Since they are often pitted against each other in the original context, fanfics play on this dynamic, showing how their competition can also be a form of connection.
In the manga, Yumeko and Ryota don't end up together. The story focuses more on the gambling scenarios and character developments rather than a romantic ending for them.