Naruto might become more competitive. For example, if he sees Hinata spending time with someone else, he'll try to outdo that person in various tasks, like training or helping villagers. He could also get moody and a bit withdrawn, not his usual energetic self. It's his way of dealing with the unfamiliar feeling of jealousy.
Naruto could start to act impulsively. He might interrupt Hinata and the person she's with, making up excuses like he needs Hinata's help with something urgent. He'll also probably try to show off more around Hinata. He'll do his most powerful jutsus in front of her, hoping to get her attention back on him. And he'll be constantly looking at Hinata when she's with the other person, with a scowl on his face.
In some fanfictions, Naruto is completely oblivious at first. He doesn't understand why Hinata is acting strange. He just goes about his day, being friendly as usual. But as Hinata's jealousy becomes more obvious, he might start to notice that something is wrong. He could be confused and try to ask Hinata what's bothering her.
Hinata might be very hurt. She's a sensitive person, so Naruto yelling at her could make her feel small and sad. She might start to doubt herself and her abilities.
Naruto might be shocked at first. He could stand there frozen, unable to believe what he's seeing. His face would probably show a mix of confusion and hurt.
He might become really withdrawn. Naruto is a very emotional person, so if Hinata breaks his heart, he could isolate himself from his friends and the village. He would probably be too hurt to face them.
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.
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 might be overjoyed. He's always been someone who values friendship and love, and Hinata's confession could be like a dream come true for him. He could immediately accept her love and start planning a future together.
He would be filled with a deep sense of grief. Naruto has always cared about Hinata, and her death would be like a part of him has been ripped away. He might isolate himself for a while, blaming himself for not being able to protect her. But eventually, he would use his pain as motivation to get stronger and fight against whatever caused her death.
Naruto could be initially in shock. Given his strong feelings for Hinata, he might not believe it at first. He could try to talk to her and ask for reasons, hoping to fix things.
Naruto might be initially heartbroken. He is a character who values loyalty and love deeply. So, when he discovers Hinata's cheating, he could be in a state of shock and disbelief.