Here is the full list of all 27 Root Letter trophies - there are 10 bronze, 10 silver, 6 gold and 1 platinum.
You got all the trophies!
Spot SHIMANEKKO 10+ times.
Select all letter replies.
All routes completed.
Finish The Bond of Marriage Chapter 10.
Finish The Bond of Marriage Chapter 9.
Complete the Cursed Letter Route, Princess of Himegamori Forest Route, A Government Plot Route, and Crossing Paths Route.
Finish The Princess of Himegamori Forest Chapter 10.
Acquired all items.
Finish The Princess of Himegamori Forest Chapter 9.
Finish A Government Plot Chapter 10.
Finish A Government Plot Chapter 9.
Finish The Cursed Letter Chapter 10.
Finish The Cursed Letter Chapter 9.
Finish Crossing Paths Chapter 10.
Finish Crossing Paths Chapter 9.
Finish Chapter 8.
Finish Chapter 7.
Finish Chapter 6.
Use the "Check" command 200+ times.
Finish Chapter 5.
Finish Chapter 4.
Use the "Check" command 100+ times.
Finish Chapter 3.
Finish Chapter 2.
Finish Chapter 1.
First conversation with SHIMANEKKO.