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