This is just speculation, but one of Tidus' lines always makes me wonder. At some point Wakka asks (even looked up the full script faq for this one):

"Wakka: "So, uh...they got fiends in Zanarkand, too?

"Tidus: "Just a few. It's a big deal when one shows up, though."

I like to think Tidus is referring to his first encounter with Sin. After all, it is a big deal when it shows up. But then again the 'just a few' part already breaks it down, so it's probably just my own make-believe. Unless he's just saying that to not sound like a weirdo from DZ...