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