At least, this is what I think.
As we know, Final Fantasy X is very heavily influenced by Western religions, Christianity in specific. And in Catholicism, they have a concept of the seven cardinal sins: pride, greed, lust, envy, gluttony, wrath and sloth.
So far, there have been six incarnations of Sin:
- The first was Yu Yevon, who:
devised a plan to both defeat Bevelle and preserve the city of Zanarkand forever. Yu Yevon called the surviving people of the city together and transformed them into fayth as conduits for a massive summoning that created a spectral version of Zanarkand from the dreams of the former Zanarkand inhabitants.
- The second Sin was Zaon, Yunalesca’s guardian.
- The third was Gandolf guardian.
- The fourth was Ohalland’s guardian.
- The fifth was Yocun’s guardian.
- The sixth was Jecht, Braska’s guardian.
But so far, we’ve only had six Sins.
To complete the concept, we’d need one more Aeon at the core of Sin. Didn’t Tidus return at the end of Final Fantasy X-2? Won’t Sin would return in Final Fantasy X-3? Tidus will disappear again in Final Fantasy X-3, won’t he?
And Tidus was Yuna’s guardian.