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