formal version : 누구십니까? informal version : 누구니? By the way, could you let me know when you are going to use this expression? I would make this sentence more natural if I know the situation where you are going to use.

Formal 실례지만 누구신가요? 실레지만 누구십니까? Less formal 누구세요? informal 누구니? 누구야?

Yes, I agree - context is very important. It's best to always provide the context if possible.