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