Kakashi would be the most perceptive. He'd see how Naruto singing 'Monster' is not just a random act but a way for Naruto to connect with his own fears and insecurities. His friends from the Hidden Leaf Village would overall be proud of Naruto for trying something new and for using the song to share a part of himself that they may not have fully understood before.
His friends might be really shocked at first. They know Naruto as a ninja, not someone who sings fanfiction. Some might find it hilarious and start teasing him in a friendly way. For example, Shikamaru might say something like 'Only you, Naruto, would come up with something so strange.'
She might be surprised at first. After all, Naruto isn't known for his singing usually. But then she could be touched if she realizes the song is for her and about their relationship, whether it's friendship or something more.
Sasuke would be furious. He already has a strong sense of justice and loyalty to Naruto. He'd likely immediately start looking for Naruto, using his Sharingan to track any possible leads. He might even go against the village rules if he thought the village was involved in Naruto's kidnapping.
His friends would be shocked at first. Sasuke might feel a sense of responsibility to find Naruto and bring him back. Sakura would probably feel guilty, especially if she was somehow involved in the situation that led to Naruto being cheated on.
Sakura would be worried sick. She might immediately inform Kakashi sensei and the rest of the village. Then, she would probably want to be part of the rescue mission. Sakura has grown stronger over time and would use her medical ninjutsu and combat skills to help save Naruto. She would also be constantly worried about Naruto's well - being and would encourage the others to move faster.
Sasuke might be initially cautious. He would observe Cell carefully to assess his threat level. He could offer his Sharingan - enhanced perception to help Naruto in the fight. Sakura, she would be worried about Naruto but also ready to jump in and heal any wounds. Shikamaru might be thinking of strategies in the background, trying to come up with a way to outwit Cell using his intelligence.
His friends' reactions would be diverse. Naruto's more hot - headed friends like Kiba might be excited about the new firepower the modern army brings. They might want to jump right into using the guns and other weapons. But Naruto's calmer friends like Neji (if he were still alive in the fanfiction) would be more cautious. They would worry about the long - term effects on the ninja world, such as the potential for mass destruction and how it would change the balance of power. And Ino might be interested in how the modern army's communication systems could be used to improve their own ninja intelligence networks.
His friends would likely be shocked and deeply concerned. Sasuke might put aside their differences and be by Naruto's side, offering his support.
Sakura might be a bit surprised at first. But she would probably be happy for Naruto as long as he is happy. She might also be curious about the elf and want to learn more about her. Sasuke would likely be more reserved in his reaction. He might not show much emotion on the surface but deep down he would be glad that Naruto has found someone. Kakashi would probably give Naruto some sage advice about making a relationship work, especially since it's with someone from a different species.
His friends would be really happy. Shikamaru would probably be like 'Well, it was bound to happen eventually, Naruto.' and offer some practical advice. Choji might bring over some delicious food for Hinata to help with her pregnancy cravings. Kiba would probably joke around with Naruto about how his life is really changing now.