Sei is the tu form of the verb essere (to be) in the present tense.
The verb forms are:
Since the speaker is talking directly to you, Italian uses sei.
Great question. Italian often leaves subject pronouns out because the verb already shows who the subject is. So Sei veramente tu? could technically just be understood from sei.
But here tu is added for emphasis.
It makes the sentence feel more like:
So the tu is not necessary for grammar, but it is very natural for emotional emphasis, especially in surprise or disbelief.
Italian word order is flexible, but this order is very natural for this kind of question.
If you said Tu sei veramente..., it would sound incomplete unless something came after it. And Tu sei veramente tu? would sound odd in most contexts.
So Sei veramente tu? is the normal and idiomatic way to say this.
Yes. Davvero is very common and would sound natural here too.
Both mean Is it really you?
Very broadly:
But in this sentence, both work well.
Italian often forms yes/no questions just through:
So Sei veramente tu? has the same basic word order as a statement, but it becomes a question because of rising intonation in speech and the question mark in writing.
This is very common in Italian.
Yes, absolutely.
Ma sei veramente tu? is also very natural and might even sound a little more spontaneous in some contexts.
Here ma does not literally mean but in the strict logical sense. In spoken Italian, it can introduce surprise, disbelief, or strong emotion.
So:
Both are natural, but they give slightly different emotional shades.