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