It should be "who ARE you?". "Who you" and "who is you" would be wrong in an informal conversation.
- Who am I?
- Who are you? (The word "you" can refer to only 1 person OR a group of people. Here, you are talking directly to the person or the group of people).
- Who is he? Who is she?
- Who are we?
- Who are they? ("They" refers to a group of many people, not 1 person. Here, you are not talking directly to the group of people, but you might be asking someone else about the group). For example, you might see a group of footballers and point at them while asking your friend "who are they?" Your friend might say "they are footballers".
I hope this helps.