For those who are curious, the author (Suzumu) listed online the contents of the letters each of the protagonists get from the "Traitor"!
Some weren't shown in the novel/manga/PVs so this may be something interesting for everyone to know.
Some of the contents are obvious but nonetheless here they are:
A-ya - "Perform Hide and Seek Alone."
B-ko - "Escape from Doppelgangers."
C-ta - "Don’t answer a call from Mearry-san."
D-ne - "Use the Monkey’s Paw, and defy fate."