Komu is the Dative case: for whom, kto is Nominative.
Nominative: Kto to? = Who is it?
Genitive: Dla kogo? = For whom?, Na kogo patrzysz? = Who(m) do you look at?
Dative: Komu daję? = Who(m) do I give to?, I give to whom?
Accusative: Kogo widzę? = Whom do I see?
Instrumental: Z kim rozmawiam? = Who(m) do I speak with?
Locative: O kim rozmawiam? = Who(m) do I talk about?
Vocative: not used