In Assassin's Creed Odyssey, the island where Kassandra or Alexios is raised is Kephallonia.
Kephallonia is the starting region for the game's playable protagonist, who grows up there after being cast away from their family. The island introduces the mercenary system, exploration, dialogue choices, naval travel, and the political tensions that shape the wider Peloponnesian War.
The player chooses either Kassandra or Alexios as the protagonist, but the character's upbringing on Kephallonia is part of the shared narrative. The island includes locations such as Sami, the Valley of Judgment, and the home associated with the protagonist's adoptive family.
Kephallonia is easy to confuse with other Greek islands visited later in the game. Naxos becomes important to the family story, while Delos and Lesbos are separate regions in the Aegean world. The game's setting spans mainland Greece and many islands, but Kephallonia is the opening home region and the place from which the protagonist begins the main journey.