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