Naruto might become more cautious. Since Kyuubi has always been a powerful and somewhat unpredictable force, he would need to be more careful in his interactions with Fem Kyuubi. His usual brash and carefree attitude might be tempered a bit as he tries to understand her motives.
Naruto may develop a more serious side. The thought of marrying Fem Kyuubi is no small matter. He would have to think about the implications for the village, his friends, and his own future. This would force him to be more mature and make decisions with more thought, rather than just acting on impulse as he often does.
He could also become more empathetic. If he starts to consider her feelings and the idea of marriage, he would need to step into her shoes. This would make him more understanding of others' emotions in general. Maybe he would start to see the world from a different perspective, not just as a ninja with a mission, but also as someone who is part of a relationship.
Naruto's core personality might not change much. He'd still be the determined and enthusiastic ninja. However, as a girl, she might become a bit more sensitive to others' feelings. For example, she could be more empathetic towards Sakura's insecurities. Also, she might be more cautious in some situations, as society in the ninja world might have different expectations of girls. But overall, she'd still have that Naruto - like spirit of never giving up.
He would likely become more irreverent. Naruto is usually quite earnest and determined, but as Deadpool, he'd be cracking jokes all the time, even in serious situations. With Fem Kyuubi around, he might tease her constantly. For example, he could make fun of her tails or her sometimes - haughty attitude. His sense of morality might also shift a bit, becoming more in line with Deadpool's grey - area ethics.
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.
One possible plot development could be that Naruto is initially shocked by Fem Kyuubi's proposal. He might be hesitant because he has his own goals and relationships in the village. However, as the story progresses, he could start to see the good in her and consider the idea. Maybe she shows him a side of her that is kind and caring, different from the fearsome Kyuubi he knew before.
One idea could be that the fem Kyuubi starts to show her affection in small ways. For example, she might protect Naruto more fiercely during battles. She could also start leaving little signs of her presence around him, like a soft growl when danger is near that only he can hear. This would build an interesting dynamic as Naruto tries to figure out what these new behaviors mean.
Her personality might become more innocent. As a Kyuubi, she was powerful and perhaps a bit menacing. But as a little girl, she could be curious about the world around her, constantly asking questions. She might also be more vulnerable and seek protection from those she trusts.
He might be more calm. Since the female Kurama could be a stable influence, Naruto wouldn't be as impulsive.
Well, when Fem Naruto wears the Kyuubi in fanfiction, her appearance can change in several ways. Her hair could become wilder and more voluminous, as if it has a life of its own, similar to the untamed nature of the Kyuubi. Also, she may have a sort of glowing aura around her, which is a manifestation of the Kyuubi's power. Her clothes might also transform to something more in line with the Kyuubi's aesthetic, perhaps with red and orange patterns that resemble the Kyuubi's fur.
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.
Initially, Naruto would likely try to find a way out of the marriage. But as time passes, he realizes that fem Sasuke is also a victim of this forced situation. They might start to communicate more, sharing their thoughts and feelings. Fem Sasuke could learn about Naruto's big heart and his dreams for the ninja world. Naruto, on the other hand, would understand fem Sasuke's past and her own desires. This mutual understanding would transform their relationship from a forced one to a more loving and respectful partnership.