Here is the full list of all 22 SYNESTHESIA achievements.
Your journey begins.
You have successfully completed Chapter 1.
You have successfully completed Chapter 2.
Miss Keller's Mini-route has been unlocked.
You hit a bad end.
You reached the end of Miss Keller's Mini-route.
You have successfully completed Chapter 3.
Isla's route has been unlocked.
You got 30 kills on Space Enforcers.
Upon Isla's testing, you got 50 kills on Space Enforcers (second scene).
You reached the end of Isla's route.
Eris' route has been unlocked.
You reached the end of Eris' route.
True route has been unlocked.
You chose the Resign Ending.
You chose the Spiral Ending.
You reached the True Ending.
You completed the True Epilogue.
You completed Eris' Epilogue.
You completed Isla's Epilogue.