Greek:
Ancient:
*Ephialtis (could be considered a persian spy)
*scetale was invented by ancient Spartans, but I can't find the inventor
*
Ieron Not a spy himslef, a tyrant. Creator of the first spy network in ancient Greece.
Modern:
*
Emmanuil Xanthos,
Nikolaos Skoufas,
Athanasios Tsakalov coofounders of Filiki Eteria, the secret organisation that organised, led and initiated the Greek War of Independence
(Alexander Ypsilantis died early. Moreover he was more a political, military figure and not a spy)
*Nikolakis Ephentis, spy during balkan wars, unfortunately there is no wikipedia sources
*
Pavlos Melas, most important participant in Macedonian Struggle.
*
Apostolos Evangelou greek spy that helped allies during WWII
Steven John Lalas, was unsuccesfull, so he might be left out
EDIT: Added Ieron as ancient spy. IMO Greeks should share the same spy pool with Byzantines