The relationship can be developed by showing how Naruto starts to see the Kyuubi not just as a source of power, but as a friend or a partner. This could be shown through small moments, like when Naruto thanks the Kyuubi after using its power to save someone. Also, fanfiction could explore how the Kyuubi's influence on Naruto changes his personality in positive ways. Maybe the Kyuubi's wild nature makes Naruto more confident and fearless in battle. And finally, they could have shared goals, like protecting the Hidden Leaf together, which would bring them closer together.
One way to develop their relationship is to put them in situations where they have to rely on each other. Say, Naruto is facing a very powerful enemy that he can't defeat alone. He has to draw on the Kyuubi's power, but the Kyuubi also has to trust Naruto to use its power correctly. This mutual dependence can strengthen their bond. Another aspect could be exploring the Kyuubi's past and how Naruto's understanding of it changes his view of the Kyuubi. For instance, if Naruto learns about the Kyuubi's origin and the reasons it was sealed in him, he might feel more sympathetic towards it, which would enhance their relationship.
In fanfiction, Naruto's relationship with the Kyuubi can be developed by having them communicate more often. For example, Naruto could start to have regular conversations with the Kyuubi in his mind. This would allow them to understand each other better.
The relationship can be developed through mutual respect. Naruto could respect the Fem Kyuubi's power and knowledge, and in return, she could respect his determination. For instance, they could start off as wary of each other but gradually learn to trust. They might share stories about their pasts, with Naruto talking about his lonely childhood and the Fem Kyuubi about her long existence. This sharing could create a connection between them.
Their relationship could start with mutual understanding. Naruto, who has always had a connection to the Kyuubi, now with a female form, could start to see her as more than just the beast inside him. They could have conversations about his dreams and her past.
One way is through shared experiences. If they are both put in a situation where they are in danger, like being trapped in a genjutsu together. They have to work together to break free. This would force them to communicate and rely on each other. Also, the male Kyuubi could start to see Naruto's dreams of peace and be influenced by them. He might then start to support Naruto more actively in his pursuits.
Their relationship could also grow through mutual respect. The female Kyuubi respects Naruto's strength and determination, and Naruto respects her power and the fact that she has been trapped for so long. This respect can lead to a friendship or even something more as they rely on each other more and more in difficult situations.
The relationship can be developed through shared battles. When they fight side by side against foes, they can start to trust each other more. For instance, if they face a powerful enemy that neither could defeat alone, they would rely on each other's strengths.
A very unique way would be if they get trapped in a time - loop together. They have to rely on each other to break free and in the process, they learn things about each other that they never knew before, leading to a deep and different kind of relationship.
Their relationship can start with mutual respect. Naruto, in his godlike form, respects the power of the Fem Kyuubi. The Fem Kyuubi, in turn, respects Naruto's newfound divinity. This respect can gradually turn into friendship as they fight side by side.
The relationship can start with mutual mistrust. Since the Kyuubi has a history of being feared, Naruto might be cautious at first. But as they interact more, they find common ground, like their shared love for the forest around the village. This simple shared interest can start to build a bond.
Their relationship could develop through shared experiences. For instance, if they go on a journey together to find a special artifact. Along the way, they face hardships like getting lost in a dangerous forest or running out of supplies. These experiences force them to work together and understand each other better.
In fanfiction, Tsunami might have a special ability that can soothe the Kyuubi's wild nature. Naruto, being the host of the Kyuubi, is amazed by this. They start to work together, with Tsunami using her ability regularly on the Kyuubi. This leads to a more harmonious relationship between Naruto and the Kyuubi, and also a deep friendship between Tsunami and both Naruto and the Kyuubi as they rely on each other more and more.