Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
Sasuke may initially be frozen, unable to process what is happening. Then, as the reality sinks in, he becomes filled with a mix of emotions. He may feel a deep sense of loss, as Naruto was his closest friend and rival. He might also feel a newfound purpose, to carry on Naruto's ideals. He would whisper words of apology and thanks to Naruto as he passes away, and this event would change Sasuke forever, making him more empathetic and committed to the village and its people.
Sasuke could be devastated. He might start crying, which is very uncharacteristic of him. He may vow to avenge Naruto's death, his eyes filled with rage and sorrow. He would also probably have a lot of regrets about their past arguments and misunderstandings. He would hold Naruto tightly, not wanting to let go as if that could keep Naruto alive.
He could also become more vengeful. If Sakura's death was caused by an enemy, Sasuke would stop at nothing to avenge her. His focus would shift solely to seeking revenge, and his already formidable skills would be honed even further in his quest for retribution.
Well, in some versions of this fanfiction, it might be that Naruto uses a powerful and forbidden jutsu to defeat an overwhelming foe. This jutsu drains his life force, and Sasuke, who has just realized how much Naruto means to him, catches Naruto as he collapses. It could also be that Naruto is poisoned, and Sasuke tries his best to save him but fails in the end, and Naruto passes away in his arms.
In this kind of fanfiction, Sasuke could die from a sneak attack. He might be so focused on his main target during the Wave mission that he doesn't notice another hidden threat. And this hidden enemy could strike him when he least expects it, leading to his death. For example, an assassin could be hired to specifically target Sasuke and use some sort of poison or a deadly jutsu that he can't defend against in time.
Well, Naruto's death would likely shatter Sasuke. He has always had Naruto as a sort of anchor, whether he admitted it or not. His whole life has been intertwined with Naruto's. Naruto's death might make Sasuke question his entire purpose. He could initially be filled with rage and a thirst for revenge, thinking that he could have prevented it. This could lead him down a darker path at first. But eventually, it might also make him realize the importance of Naruto's ideals and he could strive to carry them on in Naruto's memory.
Sasuke would be completely shattered. His world would be turned upside down. He had always seen Naruto as a rival, but also as someone important. Naruto sacrificing himself for him would make Sasuke reevaluate his entire life. He would feel an overwhelming sense of guilt and would likely isolate himself from others. However, over time, he would start to use Naruto's sacrifice as a motivation to become a better person and to protect the things Naruto held dear. He would start to follow in Naruto's footsteps in terms of values like friendship and loyalty, and would try to make amends for his past mistakes.
Sasuke could also have a complex mix of emotions. On one hand, he has a long - standing relationship with 'Naruto' as a male friend. Now that he knows Naruto is a girl, he might start to feel a different kind of connection. He may become more protective, but also a bit confused about his own feelings towards this new - found knowledge. He might even feel a bit betrayed that Naruto has hidden this fact for so long.
Kakashi's reaction would be complex. He is a man who has lost many people in his life. But Naruto was special. He would hold Naruto tightly, his hands trembling slightly. He might try to use some of his own chakra to keep Naruto alive a little longer, but deep down he knows it's futile. His face would show a mixture of pain, regret, and love. He would remember all the times he trained Naruto, all the lessons he taught him, and how Naruto had surpassed all expectations.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.
Well, if Sasuke dies in this fanfiction set in the Wave arc, Naruto would likely become more determined to avenge him. He might also grow more solemn and focused on his ninja training, realizing the fragility of life. His view of the world could shift from being a bit more carefree to a darker, more serious one as he grapples with the loss of his friend and rival.
Sasuke might be initially shocked. He has always seen Naruto as a rival and suddenly finding out Naruto is a girl could completely change his perception. He may start to view Naruto in a more protective way, as his traditional views of gender roles might make him feel the need to safeguard her now that he knows she's female.