There is no data for your current characters on your PC. What I believe was being referred to in what you read from that dev. was the list of 8 already-created characters XIV's character creator would offer you.

In Windows 7 there is a FINAL FANTASY XIV folder in My Games, where there is some character stuff (macro files, log files etc).

There seems nothing in there which contains character info (by which I mean none of the file names stand out) to which Rukkiri would be referring.

However, I do recall that the character creation tool discovered previously created characters from some file somewhere, since one time I created a character on a second PC and I didn't get the option to select a pre-created one which I normally was offered when creating characters on my main XIV PC.