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