Why not "Ki vagy"?
You here is meaning (you, plural) right ?
No, actually, this is a singular, but formal, you. "Ön".
The plural version is "Kik önök?"
Both translate to "Who are you?"
So there is no form of to be here because ön requires the 3rd person verb forms, and those are left out when describing something?
I thought the formal "you" would be translated as "maga" and "maguk". Am I wrong?
No, both "ön" and "maga" can be used for the formal you, "ön" being more polite. "Maga" is prone to being used a bit roughly. If you use it, try to be very polite.
That could be a bit disrespectful in English. Not in Hungarian?
Well, "ön" is a respectful form already but, sure, it could be said in a disrespectful way. It depends on the situation. There are definitely more polite ways.