Anima is the second of three optional Aeons that you can obtain. A fair amount of work has to be done to obtain her. You must go through each of the Cloister of Trials and complete the Destruction Sphere portion of each (to obtain the secret treasure chests).
Anima is the physical manifestation of a dream from a fayth, and in her case, it is Seymour’s mother. This is revealed during a flashback sequence during the story of the game.
Refer to the Anima side quest section for more information on how to obtain her:
Anima is one of the strongest Aeons you can obtain. Magus Sisters are arguably stronger but they can be fairly unpredictable 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.