Anima is Seymour's aeon. You can see it in a scene after the blitzball game in Luca, and you fight it in Macalania. However, you can also get this powerful aeon to fight for you.
Obtaining Anima
The details of how to obtain Anima are found on the Baaj Temple page.
Attacks
Her special attack is "Pain". This attack inflicts damage and Death to the opponent. The first time you see Anima is after the Blitzball game that you get to play at Luca when all the fiends start attacking. That is also the first time you see "Pain".
Her Overdrive is "Oblivion". Oblivion is when Anima takes you into oblivion (hence the name). Inside Oblivion, she breaks the chains that ties her hands down. It delivers hit after hit. This means that you don't need Break Damage Limit to inflict a lot of damage with this Overdrive.
When you first get her, she comes with all elemental Black Magic spells, plus Bio and Death. She also comes with a lot of status inflicting attacks, including Zombie Attack.