During the Peloponnesian War, Kosta worked as a blacksmith. As a result, his son, Supideo, unaware of his true parentage, passed his sword over to him for sharpening. Supideo had been foretold by the Pythia that he would cause the death of his mother and his father to scream and hoped to prevent this, subsequently locking himself up in a cage.
Not long afterwards, the Spartan misthios Kassandra stopped by Kosta's shop to retrieve Supideo's sword on his behalf. Kosta agreed to return the sword on the condition that Kassandra fetch him some poppies from the forest by the Red Lake.
Behind the scenes
The Eagle Bearer can engage in sexual intercourse with Kosta, resulting in the revelation that this is, in fact, the fulfillment of the prophecy foretold by the Pythia wherein Supideo would cause his father to scream.