Polish instrumental case: Kim jesteś? contrasted with Kto jesteś?1 answer
It is a good question Ania.
The meaning of Kim jesteś? points probably to a profession or a role. You can also ask Kim jesteś z zawodu? Who are you by profession?
Kto jesteś? or Kto ty jesteś? is not used so often and may refer to more general sense: Who are you as a person? I am not sure if this is even correct even thouth a popular Polish poem starts with: Kto ty jesteś? - Polak mały, Jaki znak twój? - Orzeł biały.
Regarding instrumental case in Polish language. I will probably not explain it better than here: