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
Eased a Fathers Mind
Defeated T. Bur
Read all Robot Text Logs
All missing keys used
The Show must go on
Night Theater
All Shield Upgrades
Defeated El Salserissimo
Cook the spiciest salsa
Complete Jilani'snPepper Collection
Half Music Cassettes
Found the Hidden Treasure of the beach
Cleared the Pica Pica Coliseum
Defeated Baron van Gregory
Won the Ice Skating Competition
Half HP Upgrades!
First Shield Upgrade
Defeated The Grey Kid
Half weapon Upgrades!
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!
Killed 20 Enemies
First HP Upgrade!
Won the Ant's Gang Race
Your First Bribe
Killed your First Enemy in Combat
First Music Cassette
Finish the Intro