Here is the full list of all 12 Atama achievements.
Obtained the heirloom, heard one final message...
You have escaped, but something still draws you back.
The truth has been revealed, and the curse has been broken.
You have saved the universe with your incredible shmup abilities.
Beat the game on Hardcore difficulty.
Complete the game in under an hour. (On Normal or Hardcore.)
Escape from the forest without using the heirloom. (On Normal or Hardcore.)
Use a developer commentary node.
Part of a set?
Yep, that's me. You're probably wondering how...
Obtained every journal entry in Kisaragi. (In a single playthrough.)
Obtained the actual heirloom and discovered the truth.