He could be touched. Despite his stoic exterior, Itachi may see this as a genuine and innocent gesture from Naruto, and it could warm his heart in some way. Maybe he starts to see Naruto in a new light, as someone who can bring a bit of light - heartedness into his world of shadows.
Itachi could also be amused. He might find Naruto's quirkiness endearing and the duck could be the start of a more relaxed relationship between the two. He may even start to crack jokes about the duck or incorporate it into his daily life in some small way within the fanfiction.
The duck could symbolize peace or friendship. Maybe Naruto gives it to Itachi as a sign of truce or new - found understanding between them.
Naruto might initially be shocked and angry. He has a strong sense of justice and being kidnapped goes against everything he stands for. He would likely try to fight back immediately, using his basic ninja skills and his determination. For example, he could try to use his Shadow Clone Jutsu to create distractions and escape.
Some fans might be disappointed. Fanfiction often allows them to explore different scenarios with Naruto's character. If he gives it up, it's like closing a door on those creative possibilities.
Naruto might initially be shocked or confused. For example, if they prank him with the fake ramen, he'd probably make a really funny face when he takes the first bite. But then he'd probably laugh it off because he has a good sense of humor.
In some cases, Naruto might try to reach out to the Sakura he knows deep down. He could try to remind her of their past together, their training, and their shared experiences. He believes that the real Sakura would never do this, so he tries to break through whatever is making her act this way.
Perhaps Itachi realizes that Naruto has the potential to change things for the better in the ninja world. He takes Naruto with him to train him in ways that the Leaf Village couldn't. Itachi may have some knowledge or skills that he wants to pass on to Naruto, like how to deal with the power of the Nine - Tails in a more efficient way.
Neji might get angry. He's a proud character, and being called a girl in a somewhat derogatory way (if that's how he takes it) could make him lash out at Naruto. He could give Naruto a cold stare and tell him to watch his words.
He might be really hurt. Naruto has always been an outcast and being called a demon again could reopen old wounds. So he could just stand there in shock and disbelief, his eyes wide with pain.
He might be initially shocked. Since he sees himself as a good mentor, hearing such a harsh accusation from Naruto could leave him speechless for a moment.
He might be initially shocked and confused. Since Naruto is not used to having someone else's mind in his body, he could struggle to control his actions at first.