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 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 initially shocked. Since he sees himself as a good mentor, hearing such a harsh accusation from Naruto could leave him speechless for a moment.
In some fanfictions, Naruto may become extremely bitter. He could feel a deep sense of betrayal and this might fuel his determination to become stronger, not for the sake of the village at first, but more for self - vindication. He might isolate himself from others and struggle with his identity.
This would be a huge change for Naruto. Firstly, the Sharingan's ability to see chakra flow would let Naruto analyze his opponents' weaknesses more accurately. He could potentially use this to improve his Rasengan or develop new jutsus. Moreover, in terms of combat, he could enter genjutsu battles on a more equal footing, which was something he previously struggled with due to his lack of natural genjutsu skills.
In some fanfictions, Snape might see this as an opportunity for his own growth. He might accept the apology gracefully, realizing that holding onto grudges isn't helping him. This could lead to a new relationship between them, where they start to understand and respect each other, or at least tolerate each other's presence.
Snape might initially be taken aback. He's not used to students standing up to him so forcefully. He could glare at Hermione with his cold, dark eyes, trying to intimidate her into silence.