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