When I got stuck with the names in my family tree I resorted to their meanings because names were phonetic before the 1900's searching for names that mean the same from the same area helped me. keeping with this I looked up the meaning of the name "Kostis". Kostis is used as a first name and is given this information from Parentsconnect.com (they have many name meanings): In modern Greece this is a common nickname for Konstantinos. In the Greek alphabet it is written 'kappa-omega-sigma-tau-eta-sigma', with the accent on the omega. It is transcribed as 'kostes' or 'Kostis'.
I hope that this helps you like it helped me.