in Final Fantasy X when Yuna's party is going though the old Blitzball Stadium in Zanarkand they see a memory of a mother hinting for her child to choose her to become the Final Aeon. then we see a young Seymour crying saying he doesn't want her to.
However in the Baaj Temple (where Tidas first awoke in Spira and met Rikku) is a Fayth which is revealed to be Seymour's mother who choose to become the Aeon Anima.
So is Anima a Final Aeon? If so why didn't Seymour fight Sin? if not then what happened in Zanarkand that prevented her becoming the Final Aeon but instead a regular Aeon?