"Who is this?"
Translation:Kto to jest?
Kim is in the instrumental case, which you would use for "NOUN is NOUN" constructions. So the only way "kim jest" could work is if it had an implied pronoun: "Kim on/ona jest?"
But the English sentence says "this" which is just to unspecified for "on/ona", thus it must be translated with the dummy pronoun "to", which happens to require the nominative case.