Though Yuna obtains most of her aeons automatically as part of the main storyline of Final Fantasy 10, there are three optional summons that you can obtain if you want to. One of these is Anima, best known for being the aeon that accompanies Seymour. However, obtaining Anima takes a little bit of work.
In order to get Anima, you first need to collect all six treasures hidden within the Cloister of Trials at each temple. To do this, you need to use the Destruction Spheres found in each Cloister of Trials to find the hidden treasure chest. Obtaining Anima will earn you the "Feel the Pain" achievement/trophy. We have a full guide right here on how to add her to your team.
Required Destruction Sphere Treasures
Before you head off to get Anima, make sure you have all of the hidden treasures obtained by using Destruction Spheres in the Cloister of Trials.
If you are missing any, you can always return and get them, so don't worry. However, returning later in the game means having to deal with Dark Aeon bosses in order to get back to some of the temples.
We have a full walkthrough of each temple's Cloister of Trials right here, including how to get all of the hidden treasures.
How to Return to Baaj Temple
Once you have access to the airship, you can input coordinates and passwords to visit special locations You can visit Baaj Temple by inputting the following coordinates : X: 11-16 Y: 57-63. (Airship coordinates aren't easy to pinpoint, so you might need to move your cursor within the ranges a little to find the location.)
You should recall that Baaj Temple was where you encountered the boss Geosgaeno at the beginning of the game, so this time around you can really kick his butt. Make sure you have armor equipped that protects against Petrify and Death, then save at the Save Sphere and head north to enter battle.
Geosgaeno will sometimes eat party members, ensure you use the "Struggle" option when this occurs. They'll still take heavy damage, but it'll prevent the boss from using them against your other characters.
After the battle, head south and look around the edges of the temple to find Lulu's Celestial Weapon, the Onion Knight. Afterward, you can head into Baaj Temple where there are a couple of treasure chests worth looting.
Recruiting Anima
There are six statues in Baaj Temple, each one representing one of the hidden treasures of the previous temples. If the statues do not light up in your presence, it is because you are missing one or more of the required treasures.
Once all six are lit, you are able to continue into the Chamber of the Fayth and recruit Anima to your team, arguably the most powerful aeon in the game. This will also unlock the "Feel The Pain" trophy/achievement.