Here is the full list of all 13 Inexistence Rebirth achievements.
Finish the game in "Sister" mode
Get all the skills
Obtain all equipment
Finish the game
Finish the game in hard mode
Discover 100% of the maps
Complete the bestiary
Reach level 25
Find the 10 hidden puzzle pieces
Open all chests
Getting to Claos Castle
Defeating a boss
kill 100 monsters