Here is the full list of all 26 √Letter - there are Root Letter - there are achievements.
Finish Chapter 8.
Finish Chapter 7.
Finish Chapter 6.
Use the "Check" command 100+ times.
Finish Chapter 5.
Finish Chapter 4.
Finish Chapter 3.
Finish Chapter 2.
Finish Chapter 1.
First conversation with SHIMANEKKO.
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 The Cursed Letter Chapter 10.
Finish A Government Plot Chapter 9.
Finish A Government Plot Chapter 10.
All routes completed.
Acquired all items.
Complete the Cursed Letter Route, Princess of Himegamori Forest Route, A Government Plot Route, and Crossing Paths Route.
Use the "Check" command 200+ times.
Select all letter replies.
Spot SHIMANEKKO 10+ times.