Which is correct, “Describe who you are” or “Describe whom you are”?

I am going to refine Justin Franco‘s reasoning, while agreeing with his answer.

Justin says that it’s “describe who you are”, because

We wouldn’t answer “Who are you?” with “You are him.” We’d answer it with “You are he.”

Oh really?

• the ruppes: Jesus, You are Him
• You are him by Margo
• You are him, you are the guy that raped Anna for about five years. https://books.google.com.au/book…

The explanation is not that all predicates of linking verbs in English are always nominative. A lot more people say “it’s me” than “it is I.”

The explanation is that there are two different registers of English at play here, with different grammatical rules. And, as Christopher Ray Miller’s answer has pointed out, belonging to different centuries.

Whom belongs to the centuries older variant of English, the one where people could say “it is I, Hamlet the Dane”. Whom is alien to the contemporary variant of English in which one can say “you are him”. And that is why the acceptability of “you are him” is irrelevant to the usage of whom: you don’t say “describe whom you are,” because back then you didn’t say “you are him”.