Here is the full list of all 27 Hollow Mind: The Lost Puppy achievements.
Player who completed all levels of the game
Get praise from the dog
Complete the sixth level
Find hidden toy on the 6th level
Go through the fake portal at level 6
Complete the tutorial about Metalons before level 6
Look at the Metalons
Talk to the dog about Metalons
Complete the fifth level
Use fake portals on the 5th level at least 3 times
Complete the circular maze at level 5
Find a toy hidden in a new way at level 5
Reach the portal by climbing the wall at level 5
Complete the fourth level
Find hidden portal on level 4
Get into fake portal on level 4
Complete the third level
Find hidden portal on level 3
Complete the second level
Complete the first level
Your first real portal
Collect the first coin on level 1
Complete the tutorial
Going back isn't always a bad thing!
Your first portal
The first conversation