Anima is the second of three optional Aeons that you can obtain. A fair amount of work has to be done to obtain Anima. You must go through each of the Cloister of Trials and complete the Destruction Sphere portion of each (to obtain the secret treasure chests). Check out the Secret / Optional Aeons section in the side quests area for more information on how to obtain all three optional Aeons.
Anima is certainly the strongest Aeon you can obtain as the randomness of the Magus Sisters makes them painful to use. She can use a regular attack and a move called Pain which can instantly kill enemies. Her Overdrive, Oblivion, can do a ton of damage as it hits enemies multiple times.
Anima is capable of breaking the 9,999 damage limit as soon as you obtain her.