Yuna has the ability to summon creatures known as Aeons in battle. When she does, the other two characters are replaced with the summoned Aeon, which functions as a normal party member, complete with their own HP and attacks. They are acquired when Yuna completes the Cloister of Trials at each of the temples in the game.
The Aeons can be controlled like any other party member (with the exception of The Magus Sisters), and each one of them have their own Overdrive attack. Should the Aeon die in battle, the party members that left will return. Aeons can also be dismissed with the Dismiss command, allowing the party members to return to battle once again.
Five of the Aeons are obtained throughout the game's story, while the other three are completely optional.
