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