Hinata would probably be very shy at first. She's a reserved character, so being in a somewhat exposed situation like a hotspring, especially if others are around, would make her blush and try to cover up. But as she gets more comfortable, she might start to open up and talk more freely.
Hinata would act with her usual grace. Even in the hotspring setting, she would maintain her composure. She might be a bit startled if someone unexpected like Naruto showed up. But then she would handle the situation with poise, and perhaps use the opportunity to have a deeper conversation with the person, showing her kind and empathetic side.
One possible plot could be that Naruto accidentally stumbles upon Hinata in the hotspring. Hinata is initially shy and embarrassed, but Naruto, being his oblivious self, doesn't fully understand the situation at first. Then they start having a conversation about their ninja training and goals, which deepens their bond. And Hinata realizes that Naruto has a newfound respect for her as a strong ninja.
Characters can be developed in a 'naruto hotspring fanfic' through their interactions. For example, Sasuke, who is usually aloof, might open up a bit in the laid - back atmosphere of the hotspring. He could share some of his thoughts on the future of the ninja world. This would not only develop his character but also his relationships with the other characters. Also, side characters could be given more depth. If a minor character has a fear of water, the hotspring could be a place for them to face that fear, and through that, their character can grow.
Hinata would be overjoyed. She has loved Naruto for a long time, so his proposal would be like a dream come true for her. She might start crying tears of happiness and immediately say yes without hesitation.
Naruto would probably be extremely hurt. He has always considered Hinata a friend and perhaps more. Betrayal from her would be a huge blow to his trusting nature. He might initially be in denial, not wanting to believe that Hinata could do such a thing.
One idea could be that Naruto and his friends discover a hidden hotspring during a mission. They decide to take a break there and some funny interactions occur, like Naruto accidentally slipping into the hotspring. Another idea is a more peaceful fanfic where Naruto goes to the hotspring to relax after a tough battle and reflects on his growth as a ninja.
Naruto would likely become more responsible. He's used to being the hero, but now he has to be a father too. So he might cut back on some of his risky adventures to be there for his family. Hinata would become more confident as a mother. She's always been a bit shy, but with a baby to protect, she'd find her inner strength.
He could also become very sad and withdrawn. Naruto has always valued his relationships, and losing Hinata's love would be a huge blow to him. He might start to question himself and wonder what he did wrong.
Naruto would be extremely hurt. He loves Hinata deeply and to find out she cheated with Kiba would be a huge blow to him. He might go into a state of shock and disbelief at first.
Naruto would be over the moon. He's always been a family - oriented person deep down. He'd probably start blabbing to all his friends about how amazing Hinata is and how excited he is to be a father.
Their love story in a fanfic could start with Hinata's long - held admiration for Naruto. She might finally find the courage to express her feelings more openly. Naruto, who has always seen Hinata as a friend, would be surprised but also start to notice her in a different way. He could see her strength, not just her shyness.