Another possibility is Sakura. Although she had feelings for Sasuke for a long time, in a fanfiction, the writer could change the course of events. Maybe after many shared experiences and realizing Naruto's true worth, she and Naruto got engaged secretly without Sasuke or anyone else knowing.
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.
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.
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.
It could be someone completely new, a character created just for this fanfic. Maybe he's a normal villager who has a kind heart and fell in love with Sakura when he saw her helping people in the village. He doesn't have any ninja powers but he has a great sense of humor and that's what attracted Sakura to him.
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.
It could be Misty. They have a long - standing relationship in the Pokemon series. They've traveled together and shared many adventures, so in a fanfiction, it's not a stretch to imagine her as a secret girlfriend.
It could be that a new character, someone from a far - off land that Naruto met during one of his travels, is involved in his secret life. This character could bring with them a different culture and set of skills that Naruto is learning from in secret, perhaps something that could revolutionize the way ninjas in the Hidden Leaf operate.
Perhaps in this fanfiction world, political alliances play a role. Naruto, being a powerful and respected figure, and Sona, having her own influence, their marriage is arranged to bring peace between two regions or groups. It could also be that some sort of magical or supernatural force has brought them together and fated them to be engaged.
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.