Repost:

I am hidden among many.
4 of a kind unlocks my identity.
May I be merely an option.
Rare are the ones like me.

Nope, it's not Quistis.
Althought it's extremely similar in some case, but Hidden among Many doesn't work quite well since the whole group is hidden.
Technically need to defeat 5 opponents before King.
And I agree that girls the calibur such of the likes of Quistis is rare, but that's not what I'm looking for here.

Hint: It's an Item.