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