Petros Markaris can hardly be considered a representative of Greek literature... he was culturally German (as he boasted all the time anyway) and his writings are not excactly "literature" anyway (low quality crimis... something like a less elegant and skillful John le Carre, if you know what I mean).
There are hundreds of great Greek writers and poets. Especially poets: Giorgos Seferis, Odysseas Elytes and Giannis Ritsos are three of the greatest 20th century poets internationally and they are Greek (Elytis and Seferis have both won the Nobel prize for literature).
The greatest Greek novelist is Kazantzakis. He is a compatriot of mine (we both come from Heraklion Crete) and he is the author of the well-known Zorbas (probably his worst novel, but the one most people outside Greece have read).