Here is the full list of all 21 YOMOTSU achievements.
Defeated the Necromancer after he raised three bodies or more.
Defeated the Lich without letting him taste the Forbidden Fruit.
Defeated the Hell's Guardian while its three heads are still standing.
Defeated the Mistresses after having enticed Pleasure to betray Pain.
Defeated the Black Knight on a one on one duel.
Defeated the Vampire after having touched all the bats in the car at least once.
Defeated the Aspects of the Stars in just one cycle.
Defeated the Reaper while keeping all your party alive.
Defeated the Soul Engine in less than 2 minutes.
Defeated the Fallen Angel while Light and Darkness are on her side.
Defeated the Sylphid before she flew away.
Defeated the Colossal Devil once and for all.
Got an Orb of Tunes.
Learned every skill from the mysterious girl.
Purchased all the stock from Chester.
Picked up 20 soul shards from the ground.
Found the Lost Ring of Power.
Captured the fairy in 15 seconds or less.
Used the Tainted Sparkles on an ally.
Got off the Yomotsu and returned home safely?
Got visited by That Man in your dreams.