"Who is looking for her?"
Translation:Kdo ji hledá?
Yes, the word "for" is a preposition, but it is an English preposition, and there is no equivalent preposition in the Czech sentence. The verb hledat is followed by an object in the accusative case, but it is not a verb that "takes" a preposition in Czech. But on the bright side, if it did require a preposition, ni would be the correct form.