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