The other Slavic countries would probably be very shocked. Ukraine, for example, might try to mediate between Belarus and Russia. Given their own complex relationships in the Hetalia world, they would want to keep the peace and stability among them.
America in the Hetalia world would likely see this as a huge international incident. He might try to insert himself into the situation, thinking he can be the hero and solve the problem. But his actions would probably end up complicating things further because of his over - enthusiastic and sometimes misinformed nature.
Germany, being the more serious and organized character, would probably try to approach the situation diplomatically. He would first try to understand Belarus's motives and then work towards a solution that respects the sovereignty and relationships of all the countries involved. He might try to get other European countries on board to help resolve the matter in a civilized way.
Well, one possibility could be that Belarus has some sort of misunderstanding. Maybe she thinks Russia is in danger from other forces and kidnaps him to protect him. In the context of the Hetalia universe, the characters often have complex relationships. Belarus might have her own reasons related to her loyalty or love (in a very unique Hetalia way) for Russia.
Gray would probably be one of the first to want to go after Natsu. He and Natsu have a long - standing rivalry but also a deep friendship. He'd be furious and determined to rescue Natsu. Lucy would be very worried too. She might use her celestial spirit magic to try and find Natsu's location. Erza would be very focused and strategic, rallying the rest of Fairy Tail to come up with a rescue plan.
Sakura would be extremely angry. She's Ino's friend and rival, and she would not tolerate Naruto doing such a thing. She'd probably be the first to go looking for Ino and confront Naruto. Sasuke, on the other hand, might be more calculating. He'd try to figure out Naruto's motives first before taking any action.
The villagers would be in an uproar. Naruto is a hero to them, and the idea of him kidnapping Hinata would be shocking. Some might immediately assume the worst and call for his capture. However, those who know Naruto better, like Iruka - sensei, would try to understand the situation first. They would look for reasons behind Naruto's actions and try to find a way to resolve the issue without causing too much harm. Konohamaru, who looks up to Naruto, would be deeply confused and conflicted. He would want to believe that Naruto has a good reason, but at the same time, he would know that kidnapping is wrong.
Well, Anna would be frantic. She'd immediately set out to find Elsa, not caring about the danger. She might even team up with the Guardians without hesitation. Kristoff would be right by Anna's side, using his survival skills to try and track down Pitch's location. Olaf, being the innocent and loving character he is, would be heartbroken. He might try to cheer Elsa up from afar by singing or reminiscing about their good times, hoping it reached her somehow. The trolls, if they found out, would offer their wisdom and perhaps some magical aid to help in the rescue of Elsa from Pitch's clutches.
Germany might be really confused at first. He's used to Prussia as his older brother figure, so seeing Prussia as a girl would be a big shock. He might be a bit protective in a different way than before. Italy would probably be very excited and might even start flirting a bit. He's always been a bit of a ladies' man, so he'd be intrigued by this new female Prussia.
The other members of Team 7 like Sai would be confused. Sai doesn't fully understand emotions like the others, but he would know that this is a bad situation. He would follow Sasuke and Kakashi's lead in trying to find Naruto and Sakura. Tsunade, as the Hokage, would have to make some tough decisions. She would probably send out search parties while also trying to keep the situation under wraps to prevent panic in the village. The other ninja in Konoha would be on high alert. They would be worried that Naruto's actions could be the start of a larger threat, either from within the village or from outside forces.
Annabeth would be furious. She'd immediately start planning a rescue mission. She might call on all of Percy's friends and allies at Camp Half - Blood. Annabeth would use her strategic mind to figure out where Nico has taken Percy and how to get him back.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
For creating believable characters, think about their daily lives. The Russian might have a passion for Russian football and often go to matches with friends. The male Belarusian could be involved in local Belarusian community activities. Their physical appearances can also reflect their nationalities. The Russian might have a more Slavic look, and the Belarusian could have some distinct features. Their speech patterns are important too. The Russian might use more idiomatic Russian expressions, while the Belarusian has his own way of speaking that shows his Belarusian roots.