Here is the full list of all 39 Rainbow Billy: The Curse of the Leviathan achievements.
Break the Curse of the Leviathan and save the World.
Raise ALL creatures' friendships to Level 3!
Raise 40 creatures' friendships to Level 3!
Befriend all creatures.
Catch at least one of each kind of fish!
Climb to the highest point in each area of the World.
Accept your other self and befriend Shadow Billy.
Get reprimanded for punching barrel-kind.
Collect ALL Thought Upgrades!
Peek through the window...
Collect 75 Thoughts.
Raise 30 creatures' friendships to Level 2!
Raise 20 creatures' friendships to Level 3!
Collect 50 Thoughts.
Fast travel between two distant areas of the World.
Hear each and every one of the The Confessor's secrets.
Glide using the Billy-Copter for more than 5 straight seconds!
Face your fears and befriend Calypso.
Talk to all Kickstarter Backers characters!
Own 500 coins or more.
Collect 25 Thoughts.
Discover the broken warp pipe.
Catch 10 non-gummi items while fishing.
Discover Disco Broccoli!
Fight toxic masculinity and befriend Dragalant.
Flawlessly complete 50 attack minigames!
Flawlessly complete a difficult "Dodge" attack minigame!
Flawlessly complete a difficult "Paddle" attack minigame!
Dig up 15 buried treasures.
Raise 5 creatures' friendships to Level 2!
Flawlessly complete a difficult "Circle" attack minigame!
Recolor five colors in one blow!
Collect 10 Thoughts.
Catch three different objects in one fishing session!
Convert a Thought into a Figment of Imagination!Service.
Flawlessly complete a difficult "Slots" attack minigame!
Flawlessly complete a difficult "Line" attack minigame!
Destroy 25 Objects!
Listen to Friend-Ship whistle for more than 5 straight seconds.