So a comment just sparked this thought.

Final Fantasy X doesn't let you change a whole lot of details. The obvious one is Tidus's name, which is kind of weird. But do you know why you could only change his name, and not any of the other party members?

Let me frame it this way: what other names are you able to change? That's right - the aeons.

Tidus is an aeon, or at least, a dream of the fayth.

That's why you can name him.

BAM.

QED mothersmurfers.