Here is the full list of all 21 UsoNatsu ~The Summer Romance Bloomed From A Lie~ achievements.
Loaded the game 12 times
Started the game
Cleared Chapter 1
Cleared Chapter 2
Cleared Chapter 3
Cleared Chapter 4
Cleared Chapter 5
Cleared Chapter 6
Cleared Chapter 7
Cleared Chapter 8
Cleared Chapter 9
Cleared Chapter 10
Cleared Chapter 11
Cleared Chapter 12
Cleared Chapter 13
Cleared Chapter 14
Cleared Chapter 15
Changed the main language
Opened the image gallery screen
Opened the chapter screen
Muted all character voices