Well, Naruto could initially be shocked that no one remembers. But then he might try to play it cool, because that's his nature. He could also think that they might be planning a surprise later. However, as time passes and still no one remembers, he would start to feel really down. He might even go off on his own to brood about it, wondering if he's not as important to his friends as he thought.
Iruka would be inconsolable. He was like a father figure to Naruto in the early days. His death on his birthday would bring back all the memories of Naruto's growth and how far he had come. The village as a whole would go into mourning. Naruto was the hero who saved them multiple times. There would be a sense of hopelessness among the villagers, and they would struggle to find a new source of inspiration without Naruto around.
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.
At first, Naruto might be in shock. He has always trusted his friends, so the sudden betrayal would be a huge blow. He could be silent, unable to process what has just happened.
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.
There could also be friends who are so loyal to Naruto that they would consider following his request if they truly believe it's for the greater good. However, they would be deeply pained by it. Kakashi, for instance, might understand Naruto's selfless nature but would also struggle with the moral and emotional implications of taking his life.
Sakura might be a bit surprised at first. She would probably tease Naruto a little about it, but then she'd also be curious to see what's in those fanfics. She might even start reading some herself to see how the fans portray her relationship with Naruto.
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.