Aeons are divine creatures that only answer the call of a true summoner. Each aeon protects its master with unique powers and characteristics. Aeons grow stronger as the summoners do.
When Yuna summones an aeon, the other party members must withdraw to a safe distance. Only then can Yuna direct the aeon's actions. Like your regular party members, aeons have an Overdrive gauge, and when it charges up thhey can unleash an awe-inspiring Overdrive technique. If an aeon's HP reaches zero, it will vanish and the other characters will return to continue the fight.
Vallefor- A swift, flying aeon.
Ifrit- An aeon engulfed in eternal flame.
Shiva- An icy aeon in the form of a beautiful woman.
Bahamut- A unbelievable strong aeon with brute strenght and powerful black magic.