According to Assassin's Creed Odyssey's canon, Kassandra is the Eagle Bearer.
Odyssey lets the player choose Kassandra or Alexios as the playable character. The game's canon novelization and later franchise continuity identify Kassandra as the historical protagonist who carries the title Eagle Bearer. Alexios instead fills the role of Deimos, the opposing sibling, in that canonical version of the story.
Kassandra is a Spartan-descended mercenary who travels through the Greek world during the Peloponnesian War. Her story includes Athens, Sparta, the Aegean islands, and encounters with members of the Cult of Kosmos. She also uses the Spear of Leonidas, an important artifact linked to her family.
Because the game offers player choice, many players remember Alexios as the protagonist of their own playthrough. That does not change the canon designation used by the wider series. The answer therefore depends on the official canonical version rather than on every possible player-selected avatar.