One of the key features of the game is the Aeons, which are the summoned creatures Yuna can use in combat. Five of the Aeons will be obtained during the main story, while the remaining three are completely optional. All of the Aeons are fully functional party members, complete with their own stats and abilities. Yuna is able to summon them at will during battle, as they don’t cost MP. All of the Aeons, similar to the normal characters, have their own Overdrive gauges, although they will increase upon either dealing damage or taking it.
Whenever Yuna summons an Aeon during battle, all of the characters on the field will leave and it will just be the Aeon. As mentioned above, all of them have their own abilities at their command. However, they do share some similarities to one another, like the Shield and Boost commands. By pressing right on the directional pad, you can choose one of the two above commands. Shield will halt the progress of the Overdrive gauge, but the Aeon will take less damage while it’s active. On the other hand, Boost will expedite the Overdrive gauge, at the cost of taking more damage.
The last command on this list, Dismiss, will send the Aeon away, although there is no cooldown on summoning it again. Should an Aeon’s health reach zero, then the Aeon will be dismissed and you will need to do one of two things before they’re available again. You will either need to touch a Save Sphere (completely refills the Aeon’s HP and MP), or participate in a set number of battles, automatically bringing the Aeon back. The number of battles will differ, depending on the downed Aeon.