The number zero (0) does not really have a Korean native numeral of its own. In fact, the only way to say zero in Korean is by Hanja: Yeong (영) or Gong (공).
I'm very tempted to say that it's because the whole concept of zero had never existed in the Korean numeral system, considering the fact that it also wasn't really part of the Roman Numerals, as the Romans used the Latin word "nulla" (which means "none") instead of a Roman numeral alongside their mathematics. So that means... the Koreans had used something like "없음" (which means "absent" or "nonexistent") instead of their own numeral? The number zero wasn't actually invented back then.
(Also, on a side note: in modern/contemporary days today, when writing (alongside?) Roman numerals, instead of writing the Latin word "nulla", we write the Arabic numeral 0, which is a real number and not just a symbol that represents nothing, in order to indicate zero. So 0/1/2/3/4 becomes 0/I/II/III/IV.)
Anyway, the point here is: Why didn't the number 0 actually have a Korean native number of its own?