Here is the full list of all 30 Root Letter Last Answer achievements.
Speak to Shimanekko for the first time.
Finish Chapter 1.
Finish Chapter 2.
Finish Chapter 3.
Finish Chapter 4.
Finish Chapter 5.
Finish Chapter 6.
Finish Chapter 7.
Finish Chapter 8.
Finish The Bond of Marriage Chapter 9.
Finish The Bond of Marriage Chapter 10.
Finish Crossing Paths Chapter 9.
Finish Crossing Paths Chapter 10.
Finish The Princess of Himegamori Forest Chapter 9.
Finish The Princess of Himegamori Forest Chapter 10.
Finish The Cursed Letter Chapter 9.
Finish Finish Finish The Cursed Letter Chapter 10.
Finish A Government Plot Chapter 9.
Finish A Government Plot Chapter 10.
Complete the Cursed Letter Route, Princess of Himegamori Forest Route, A Government Plot Route, and Crossing Paths Route.
Acquire all items.
Finish The Cursed Letter Answer Arc, A Government Plot Answer Arc, and The Princess of Himegamori Forest Answer arc.
Use the "Check" command 100+ times.
Use the "Check" command 50+ times.
Select all letter replies.
Finish The Cursed Letter Answer Arc.
Finish A Government Plot Answer Arc.
Finish The Princess of Himegamori Forest Answer Arc.
Finish The Bond of Marriage Sequel Arc.
Finish all routes.