Who you are? may be an acceptable variant of Who are you? in African American Vernacular English or an offshoot thereof.
African American Vernacular English is defined by Wikipedia as:
"a variety (dialect, ethnolect and sociolect) of American English, most commonly spoken today by urban working-class and largely bi-dialectal middle-class African Americans."
Among many other idiosyncrasies, this vernacular allows altered syntax in questions. Wikipedia gives the following examples:
Altered syntax in questions: Why they ain't growing? ('Why aren't they growing?') and Who the hell she think she is? ('Who the hell does she think she is?')
Notice that in "Why they ain't growing?" the direct object pronoun they comes before the copula ain't. This is very similar to what's happening in "Who you are?"
Another idiosyncracy in this vernacular is the use of the copula be in non-standard ways. For example, the copula often occurs uninflected, or inflected for the wrong number, or even dropped. For example, in "He be crazy", the copula is uninflected; in "They is crazy", the subject is plural but the copula is singular; in "He crazy", the copula is dropped entirely.
While it may be that the more common vernacular variant of "Who are you?" is "Who you be?" or "Who you is?" or even "Who you?", I believe that I have heard "Who you are?" before, and I can easily imagine someone who speaks the vernacular uttering it. I will try to find some data corroborating this, but the difficulty in finding such data suggests that the be and is variants are far more common than the are variant.
As a final caveat, I advise English Language Learners (as well as native English speakers who have not grown up with this vernacular) not to try to imitate it. It can be considered offensive, as evinced by this passage:
Just as it is offensive for people to don bindis, kimonos or Native American headdresses without belonging to those respective cultures, it is offensive... to mimic African-American dialect in order to sound “cooler”.