Here is the full list of all 20 A Sky Full of Stars achievements.
Complete the Hikari route.
Complete all routes.
Watch all of the cutscenes.
Watch 80% of the cutscenes.
Complete the Saya route.
Complete the first common route.
Complete the Korona route.
Watch 60% of the cutscenes.
Complete the game for the first time.
Complete the Orihime route.
Watch the ending movie.
Watch 40% of the cutscenes.
Complete the second common route.
Watch the second opening movie.
Watch 20% of the cutscenes.
Watch the first opening movie.
Watch a cutscene for the first time.
Start the game.
Open sound screen.
Open the gallery.