Here is the full list of all 18 Come Together Demo achievements.
Complete the Tutorial
Unlock your first Inventory entry
Make a first purchase from Merchant
Successfully dodge all the falling red blocks
Help Violetta with "Flappy Square" game
Help Limy with sorting all the Boxes
Find Tommy's Stopwatch and give it to him
Beat all of Tommy's challenges
Finish the main game (Levels)
Find a Secret
Unlock all Inventory entries
Finish "The Platformer" game mode
Get a score of 20 or more in "Flappy Square"
Come Together 30 times
Die 15 times
Die to a Box
Buy all items from Merchant.
Fully finish the game