She is there.
Why is she there?
This is a direct question. In direct questions, subject and verb switch places.
She is there.
Tell me why she is there.
The last example is an indirect question. In indirect questions, subject and verb are not switched: the normal word order is preserved. The only thing with relative and interrogative subordinate clauses is that the interrogative or relative pronoun (in this case, why) must come first.
She is gay.
Who is gay? (who stands for she, but it's not
is who gay? , because who must come first)
Tell me who is gay.
Who replaces she, the subject; the normal word order from she is gay is preserved in the indirect question ( Tell me who is gay. ) as expected. In the direct question ( Who is gay? ), the word order is only preserved by accident, only because who must come first.
Cleopatra is my lover.
Who is Cleopatra?
In this case, Cleopatra is the subject, my lover the subject complement. The word who stands for my lover. The word who must come first*, so it starts with who. It is a direct question, so subject and verb are inverted ( Cleopatra is → is Cleopatra ).
I don't know who **Cleopatra i**s.
This is an indirect question, so there is no inversion: it's Cleopatra is. The interrogative pronoun who still has to come first.
If you had a sentence like this:
Gay is my friend. (normal word order)
Then it would go like this:
Who is Gay? (subject and verb switched)
Tell me who Gay is. (normal word order)
Then gay would be the subject, so it would have to be a noun; but there is no noun gay, so the only possibility would be the name of a person, with a capital: someone named Gay. This is not what you mean, so this is simply incorrect in context.
My friend is gay. (normal word order)
Who is gay?
In a direct question, like this one, word order would normally be inverted, so
is who, except that who must come first, so the inversion becomes invisible.
Tell me who is gay. (normal word order)
In the indirect question, we have the normal word order, as expected.
Tell me who are you.
This is just wrong. It is an indirect question, so this would have to be the normal word order. Then the basic statement should have been this:
Someone are you.
This is wrong. Alternatively, someone might have been somewhere else in the original statement (because who must come first, it may obscure the original position of someone):
Are you someone.
This is also wrong. As a direct question, it would work; but this would have to be the original statement, and in a statement you can't start with the verb like this. So your
tell me who are you is not possible.
Tell me, who are you?
This is possible, because it is a simple direct question, because of the comma and the question mark. It has the same word order as when you leave out tell me:
You are my friend. (normal word order)
Who are you? (inverted)
It is a direct question, so subject and verb are inverted. The word who must come first.