As I've already said elsewhere, in Greek the full form would be "Kostas," but, as the "s" (or rather "sigma") is only used in the nominative (or subject) case, it tends to be regarded as tacked on. ("Kostas said," but "Kosta's house," "give this to Kosta" or "I see Kosta.") This is why Greek forms of Hebrew names like "Elijah," "Jonah" and "Judah" add the "s" ("sigma") ("Elias," "Jonas," "Judas")
Actually, Bananarama, I must disagree. Kosta is a very strong name, in my opinion, and entirely masculine. It is an appealing and attractive name, without being too childlike (considering that it is, in fact, mainly a nickname).I have found it been in use by many Greeks, though as the form of "Costa", instead of a "K". More than that, it would have been shortened for Konstantinos (or a similar form). Regardless, I have also heard of it as a first given name.