It could be Hinata. In the original series, Hinata has a deep affection for Naruto. In a fanfiction, it would be a natural progression for her to be his secret fiance. Maybe they got engaged secretly to avoid some political unrest in the village.
A possible plot twist could be that his secret fiance is actually an enemy in disguise. She got close to Naruto under false pretenses, and as the story unfolds, Naruto has to deal with the betrayal and the fact that his feelings were used against him.
I'm not sure which specific 'False Fiance Secret Story' you are referring to. There could be many stories with such a title or concept. It could be about someone who pretended to be a fiance for some ulterior motives, like financial gain or to deceive family and friends.
Naruto and Rias are the central characters. Naruto is known for his never - give - up attitude and his array of ninja techniques. Rias is a complex character with her own political and social standing in the devil world. However, in some fanfictions, other characters from both the Naruto and High School DxD universes can become important. Kiba from Naruto might befriend Naruto in the new world and help him understand the devil society better. And from the High School DxD side, Akeno, Rias' friend, could play a role in either supporting or testing Naruto's relationship with Rias.
Well, it could be a fanfiction where Naruto discovers he has the Mokuton ability in a very unexpected way. Maybe he gets it through some ancient power transfer or a connection to a long - lost clan. And this new power of Mokuton could lead him to face new enemies or form new alliances. It might also explore how his friends react to this new side of him.
It could be that Naruto is secretly training a new and powerful jutsu that he doesn't want anyone to know about yet. Maybe he's afraid that if his enemies find out, they'll try to stop him before he perfects it.
It might also be Gaara. Their initial encounters were filled with tension, but as the story could be twisted in fanfiction, that could develop into a different kind of relationship. Gaara's own journey of self - discovery and Naruto's influence on him could be a basis for a romantic relationship in fanfic.
Another plot could be that the false fiance is part of a revenge plan against the main character or their family. By getting close as a fiance, they can execute their plan more effectively, like causing a rift in the family or ruining the main character's reputation. The story would be full of twists and turns as the real intentions are gradually uncovered.
A common plot in Naruto Rias Fiance fanfiction is the power - up journey of Naruto. As he is now in a new environment with different power systems, he has to level up. He could gain new devil - related powers through his connection with Rias. Also, there could be a plot about their relationship development. How they overcome misunderstandings and cultural differences. For instance, Naruto's straightforward ninja way might clash with the more complex and often secretive devil society, and how they resolve these issues can be an exciting part of the story.
It could be a random villager that Naruto met during one of his travels. Maybe she was from a small village that Naruto helped protect, and they had a brief encounter that led to the child.