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