Here is the full list of all 31 STAY achievements.
And then there was light...
Who are you, stranger?
Don't leave during a time interval
Never kill the cat
A never-ending book which is not for Bastian
A life within a life within a life within...
Stare at the full moonlight with Quinn
Guardians, feathers and the unknown
Board game solitaire
Hail to the chief
Never leave the magic circle
Don't lose sight of the north
Bad hair day not allowed
Play it cool
Memories shaped into bits and pieces
Dance with the ugliest partner
Cracking the final code
Reach the final chapter with the highest emotional bonding
Reach the final chapter with the highest level of trust
End Quinn's journey with a feeling of Acceptance and Hope
End Quinn's journey with a feeling of Bargaining
End Quinn's journey with a feeling of Anger
Back to square one
End Quinn's journey with a feeling of Isolation
End Quinn's journey with a feeling of Denial
End Quinn's journey with a feeling of Depression
Become the perfect soulmate and never leave Quinn
Reveal all possible endings
Discover all possible items
Unveil all secret rooms
Collect all the other achievements