Here is the full list of all 44 Underhero achievements.
All Music Cassettes
You deserve a raise
Defeated ???
The End of The World
Defeated Mr. Stitches
Spared the Masked Children in Heroes Road
All HP Upgrades!
Bribed 20 Enemies
All weapon Upgrades!
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
Defeated El Salserissimo
Cook the spiciest salsa
Half weapon Upgrades!
Complete Jilani'snPepper Collection
Found the Hidden Treasure of the beach
Half Music Cassettes
Cleared the Pica Pica Coliseum
Defeated Baron van Gregory
All Shield Upgrades
Defeated The Grey Kid
Won the Ice Skating Competition
Half HP Upgrades!
You gave Blobber his favorite drink
Made your first drink
Completed the Bros Quest
First Shield Upgrade
Defeated The Moth Queen, Alexandria II
A tree full of beer
First weapon Upgrade!
Killed 20 Enemies
Won the Ant's Gang Race
First HP Upgrade!
First missing key used
First Music Cassette
Killed your First Enemy in Combat
Your First Bribe
Finish the Intro