who are you- is correct.

It should be "who ARE you?". "Who you" and "who is you" would be wrong in an informal conversation. Remember this: - 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.

The pronoun 'you' is always treated as plural. So you need to use 'are' instead of 'is'. Who are you? 'Who you?' sounds very bad.

You wouldn’t use either of them. You would say, “Who are you?"

Both are wrong and neither of them are used, however informal the situation. The only time I can think of when "to be" is omitted is when it's part of a yes-no question in the second person continuous e.g. "(Are) you doing...?". It's very informal and should be avoided in formal situations. Otherwise "to be" is never omitted.