[Used Can]

The author of those letters was also probably in Rokkenjima during the meeting. Since once or twice (in EP2 and EP3, unless I'm mistaken), the key for the vaults was written with red letters in the wall.

I wonder what are the chances for the one who wrote that key, being the same person who drew those magic circles. In Gohda's TIPS, we learn those magic circles had been used even before the family meeting, and it seems the servants are related to those. In fact, in that TIPS, it seems as if the servants are hiding something/someone.