Giorgio Adonis Vellek is the son of Doctor Paul Vellek who lives in Greece.

In "Feast," his house on Atokos island is raided by a team from the Nathan James in hopes of finding clues to the location of Vellek's lab. At the same time, Omar Bin Dalik, seeking revenge upon Giorgio, attacks as well. Recognizing a common enemy, Giorgio provides the Americans with more powerful weapons and Omar is killed by a bomb set by Lt. Carlton Burk. However, Giorgio escapes custody to return to his father's side. He is caught in the explosion and the right side of his face is subsequently scarred from the burns.

In "Lazaretto," James Fletcher is shocked to learn the true extent of Vellek's plans and tries to convince Giorgio, who displays doubts, to turn on his father without success. Giorgio catches Fletcher in the process of broadcasting a warning message to the Nathan James, having betrayed Fletcher to his father. Giorgio then kills Fletcher for his actions and supervises Fletcher's body being dumped overboard.

In "Detect, Deceive, Destroy," after the HS Proteus is struck by missile fire from the Nathan James, he is mortally wounded by a piece of shrapnel and dies in his sister's arms.