I am not an expert in English grammar.
Grammatically speaking, though, shouldn’t it be correct to say “who is you?” instead of saying “Who are you?”
For singular nouns we use “is”, like : “where is he?”, “Where is it?”; “The car is rolling.”
For plural nouns we use “are” like – “where are they?”; “what are they?”; “The cars are rolling.”
Why the “are” in “who are you?”
When we refer to a group of people – like “you guys” – we can say “where are you (guys)?”
Apparently the “are” in “who are you?” is used because the “you” that we use in everyday conversation is the formal or plural form of you.
Formality vs. Familiarity
In French – there is a single form of the word “you” – “tu” in French – for people you are familiar with and a more formal form or plural from of “you” – “vous” in French – for people a certain honor is due, and sometimes a distance is due.
Apparently it is like in old English the difference between thou (you for a single person) and ye (you for a group – thus the declaration – “Hear Ye! Hear Ye!). When we ask who are you – We are in essence saying “who art ye?”
“So?” you are probably asking – “Who cares?”
And rightfully so.
Proper Relationship with Parents
There are people that we are close with and people with which we maintain a distance.
In the Torah when Yaakov (Jacob) and Esav (Esau) were to receive their blessings from their father Yitzchak (Isaak) each addressed their father in a different form.
Esav refers to his father in the more distant form – the third person form. (Bereishit / Genesis 27:31)
Yaakov refers to his father in the more familiar form of you. (Bereishit / Genesis 27:19)
As we follow the teachings of Jacob, the Torah teaches that our parents are to be addressed by the familiar form – meaning they are a close, and integral part of our lives – not a distant relative to respect. Children too are treated as close ones.
Hash-m is Always Close
When we address G-d as well we use the familiar form of you – atah in Hebrew – showing that Hash-m is an integral and close part of our daily lives.
Every moment Hash-m is close and available to hear our prayers. We just have to call out to Him.