"But who are you?"

Translation: लेकिन तुम कौन हो?

[DennisLDuolingo]

Shouldn't कौन be in the oblique form (किस), since तुम is the subject?

[Renaissance_Man.]

Indian philosophy anyone?

[_FJ_]

Is it correct to say like this; कोन हो तुम?

[ph.pLs4eQ]

Yes

[ph.pLs4eQ]

It is correct but it seems a bit rude to say

[ph.pLs4eQ]

Yep its correct