Here is the full list of all 39 Swords Souls: Neverseen achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Complete endless stage 120
Complete endless stage 100
Complete the museum to 100%
Get all Soul Master medals in all training areas
Complete endless stage 80
Beat the final boss using only beginner armor and weapons
Complete endless stage 60
Find all lost children
Catch 11 different types of fish
Tame all pets
Get three Combo medals of rank Soul Master
Juggle with the same coin 30 times
Juggle with the same coin 20 times
Complete endless stage 40
Become friends with all mercenaries
Tame 8 pets
Become Soul Master in all training areas
Become friends with two mercenaries
Complete endless stage 20
Get three Combo medals of rank Platinum
Defeat chapter 6 boss
Become friends with a mercenary
Defeat chapter 5 boss
Get three Combo medals of rank Gold
Trigger the Oof ring once
Catch 8 different types of fish
Tame 3 pets
Get 3 perfect in a row while fishing
Juggle with the same coin 10 times
Defeat chapter 4 boss
Defeat the Legendary Hunters
Defeat chapter 3 boss
Get three Combo medals of rank Silver
Defeat chapter 2 boss
Catch 3 different types of fish
Defeat chapter 1 boss
Get three Combo medals of rank Bronze
Defeat the Mama Mole
Get three Combo medals of rank Wood