ANIMA
Anima is the dark aeon she is a secret aeon and you may notice she was the aeon that seymore used in luca stadium, but you will find out more about that later in the game. you can get her by getting all the secret items in the game (even in zanakand). then you can get anima at the baaj temple (were you start in spria) you can only get this aeon when you get the airship.This aeon's overdrive is oblivion and when you get her it does about 70,000 damage, you have to fight the monster that trys to eat you at the staqrt of the game if you want her but thats easy and worth it :)
BAHAMUT
Bahamut is the strongest "normal" aeon in the game and is alao the last you get. he has a damage break limit already you get him at the temple in Bevelle, personaly that is the hardest temple in the game, his overdrive is mega flare and when you get him it does about 17,000 damage useful when fighting seymore.
Ifrit
ifrit is the second aoen you get at kilika temple he isent very strong until later in the game. he is good vs. the fire things on the mushroom rock road, since they just heal him. use him vs the chokobo eater on the highroad, you will need velfor on that too. His overdrive is hellfire and it does about 4,000 but goes up rapidly from that.
Ixion
ixion is the lightning aeon, it is very powerful later in the game when you get the sprit lance. his overdrive is Thor's Hammer it does about 7,000 when you first get him but it depends on your yuna and how advanced she is (if you follow the do's and dont's she should be very powerful) Ixion has a powerful attack power and has a strong attack called "Aerospark" his evade is very good.
Magus sisters
The magus sisters are three aeons in one and are the most powerful in the game (thats why they are hidden, look in the hidden page to find out how to get them) Sandy: The tall one is the strongest with phisical attacks like razzia the strongest attack in the game (will be the first attack to reach 99,999) Mindy: The Short One uses attacks like passado that get really powerful after a while Cindy: the big boned one she is good at healing and casting magic (she takes alot of brakes) Delta Attack: The sisters overdrive all three have to get there overdrive meeters up and it is the most powerful attack in the game (it will kill overdrive sin easy)
Shiva
Shiva is the ice aeon that you get before the first seymore battle at macalania temple, because of her great speed if you user her there it helps (there her name is ????) her overdrive is diamond dust and it does about 7,000 depending on the strenth of your yuna.
Velfor
velfor is the first aeon you get she is not very strong until you get "Energy Blast" (look at hiddden page) she has the basic black magic and energy ray does about 2,000 damage at the start
Yojimbo
Yojimbo is a secret aeon that you have to pay, you get him after going therw the cavern of the stolen fayth he is a really strong aeon but for every turn you must pay him or he dismises himself, some hate yojimbo and some like him its all up to you he only has four moves one of them is rare (more info go to hidden page)
