There could be a jealous character, maybe another ninja who has feelings for Naruto. This character would see the fem Kyuubi's possessiveness as unfair and would try to break their bond. They would create problems for the fem Kyuubi and Naruto, acting as the antagonist in the fanfiction.
It could be other powerful ninja clans who see the fem Kyuubi's possessive behavior towards Naruto as a threat to their own power and influence. They might try to separate them.
Obviously, the fem Kyuubi herself could be the main antagonist. Since she is the one manipulating Naruto, her actions are causing problems and driving the conflict in the story.
It could be an evil ninja who wants to capture the fem Kyuubi for her power. He might see the unique bond between Naruto and the fem Kyuubi as a threat to his plans of world domination.
The plot might center around the fem Kyuubi's desire to own Naruto due to some sort of pre - determined fate. She might have a vision or a prophecy that Naruto is essential to her future. So she attempts to control his actions and decisions, which Naruto initially resists. This creates a dynamic tension between them as they navigate through different challenges in the Naruto universe.
Another interesting one could be 'Fem Kyuubi's Desire for Naruto' on Archive of Our Own. In this fanfic, the fem Kyuubi is portrayed with a lot of depth. It shows her inner thoughts and feelings towards Naruto as she tries to navigate the world around her while being attached to Naruto in a very special way. It's a really engaging read if you like this concept.
It could be an ancient shinobi who once tried to control the Kyuubi but was sealed away. Now, with Naruto as the new host, he sees an opportunity to take over the Kyuubi's power again.
Well, in Naruto fanfiction, the concept of 'fem Kyuubi owns Naruto' can be interpreted in various ways. It could mean that she has a dominant role in his life, perhaps guiding him or having a special connection with him. But again, different fanfictions will handle this idea very differently. Some might show it as a literal ownership in a magical or spiritual sense, while others might use it more metaphorically.
It could be a rogue ninja who has a vendetta against the Hidden Leaf Village and specifically targets Naruto and Fem Katsuyu because of their unique partnership. This ninja might have some dark jutsu that can counter Fem Katsuyu's abilities.
Madara Uchiha could also be a main antagonist in this fanfiction. He has grand plans for the ninja world and the released Kyuubi could be a key part of his scheme. Since Naruto is dead and can't oppose him, he might try to use the Kyuubi to reshape the world according to his vision. He has the power and the ambition to be a major threat in this situation.
It could be Orochimaru. He's always been interested in power and the secrets of the Kyuubi. With Naruto releasing the Kyuubi fuuinjutsu, Orochimaru might see this as an opportunity to either steal Naruto's power or use him for his own evil experiments.
Another way could be through communication. In some private moments, she might open up to Naruto about how she feels. She could say things like 'You are important to me, and I want you by my side always' in a soft and sincere tone.