Here is the full list of all 28 Ralf achievements.
Clear the Tutorial Level
Clear the first level
Drink an Energy Drink for a Sugar Rush Power Boost
Use your Blueberry Blaster to defeat an enemy
Use your Green Apple Throw to defeat an enemy
KO an enemy by jumping on their head
Defeat an enemy boss
Open the Shop where you can purchase Energy Drinks and Cake Slices to help on your adventure
Make a purchase in the shop
Free a captured Blobber by breaking open a cage
Collect all 100 candies in any level
Clear any level with full health
KO all enemies in any level
Free all 5 captured Blobbers from their cages in any level
Collect the Green Key
Collect the Blue Key
Collect the Purple Key
Collect the Red Key
Collect the Orange Key
Collect the Yellow Key
Defeat the final Boss
Collect all 100 candies in all levels
Clear all levels with full health
KO all enemies in all levels
Free all 5 captured Blobbers from their cages in all levels
Complete all the objectives in extra stage 1
Complete all the objectives in extra stage 2
Complete all the objectives in extra stage 3