/shared2/menu/FaceLib/RFL DB.dat

Please note that the file name is RFL_DB.dat, the wiki seems to have removed the underscore.

Anyways, this is the database-file where Mii's are stored. It's only a signature and then Mii entries, they look exactly as they did in the Wiimote, you can extract your Mii's from this file and open it up on your PC in those Mii-editors.

Any modification (even valid modifications like changing height) causes an error to appear on the screen. This error does not occur when running in Maintenance Mode, but it is the same kind of message like the one that says "Wii in Maintenance mode, Message Board will not be available.", so it could just be a bug or something.

I would say it's safe to mess around with this file though, because they have a reset function, so you can reset your file back to its default state (remove all Mii's) if something goes wrong. I made a backup and I was able to restore it when the error appeared.

I've tried the following Wii-games which are dependent on Mii's, while the database was invaild, Metroid Prime 3: Corruption and Wii Sports. (feel free to add to this list) Both games gave me an error, and returned to the Wii Menu.

I don't know how they notice that the file is changed, maybe it's signed or maybe I did something wrong (but restoring worked so..).