Here is the full list of all 44 Underhero achievements.
First HP Upgrade!
First Music Cassette
Killed your First Enemy in Combat
Finish the Intro
First weapon Upgrade!
Half weapon Upgrades!
All weapon Upgrades!
Half HP Upgrades!
All HP Upgrades!
Half Music Cassettes
All Music Cassettes
First Shield Upgrade
All Shield Upgrades
First missing key used
All missing keys used
Your First Bribe
Killed 20 Enemies
Bribed 20 Enemies
Won the Ant's Gang Race
A tree full of beer
Defeated The Moth Queen, Alexandria II
Made your first drink
You gave Blobber his favorite drink
Completed the Bros Quest
Defeated The Grey Kid
Won the Ice Skating Competition
Defeated Baron van Gregory
Cleared the Pica Pica Coliseum
Complete Jilani'snPepper Collection
Found the Hidden Treasure of the beach
Cook the spiciest salsa
Defeated El Salserissimo
Night Theater
Read all Robot Text Logs
The Show must go on
Defeated T. Bur
Recruited the Moth Queen
Recruited The Baron
Spared the Masked Children in Heroes Road
Defeated Mr. Stitches
The End of The World
Defeated ???
You deserve a raise
Eased a Fathers Mind