Originally Posted by
Jiro
The "sacrifice" that a guardian endures to become the Final Aeon is to become a fayth. Fayths are able to impart their wisdom on whomever they choose. Anima is not Yuna's Final Aeon because their bond is not strong enough to draw out Anima's true power. Jecht, too, would have been summonable were Yuna to pray to him, but I doubt their bond would have been strong enough either. Now if Tidus were to gain the ability to summon Jecht, then it probably would've been powerful enough of a bond to be the Final Aeon - if you discount the fact he was already successfully summoned and defeated Sin.