lolwat. What's missable before the opening sequence? Spoiler tag it if you need to
I believe it's talking to the person hiding in the moogle suit when you have control of Rikku after the opening battles. EDIT: On Luca docks that is.
It's later revealed to be Yuna wearing that so she doesn't get seen when she's supposed to be on stage during the concert. Letting the moogle heal you gives you a bonus scene after the optional mission in Luca which adds to your %.