Purchase Dissidia 012: Duodecim [prologus] Final Fantasy from the PSN store. At the installation screen, download the file with the the data for Aerith to your Memory Stick. Start Dissidia 012: Duodecim Final Fantasy to unlock Aerith as an assist character.
During the tutorial you will be asked by the Moogle if you have already mastered Dissidia. Answer "Yes" to battle against a level 130 Feral Chaos with a level 1 Lightning. If you defeat it, it will become unlocked in Free Battle mode.
In the untold tale, you will find a 20 floor gateway attempting to save a moogle (Cid "the creator"). If you clear the gateway with Tidus (who know you have if Tidus is in the the cutscene with the moogle), either of the two bravery attacks he has (the ones where he throws a ball) will sometimes appear as a moogle ball.