The interrogative pronoun is non flectional.
Here, all three genders are are represented by an own expression.

• ποιός / ποιά / ποιό;
  → Who?
  Ποιός είναι στο τηλέφωνο;
  → Who is on the phone?
• ποιόν / ποιά / ποιό;
  → Who/whom?
  Ποιόν ψάχνετε;
  → Who are you looking for?

In Greek, some interrogatives are flectional, such as:
πώς (how), γιατί (why), πού (where), από πού (wherefrom), πότε (when), τί (which), τί είδους (wherefor/for what).