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 HP Upgrades!
All Music Cassettes
Bribed 20 Enemies
All weapon Upgrades!
Recruited The Baron
Recruited the Moth Queen
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
Complete Jilani'snPepper Collection
Found the Hidden Treasure of the beach
Half Music Cassettes
Cleared the Pica Pica Coliseum
Half weapon Upgrades!
Defeated Baron van Gregory
Defeated The Grey Kid
Half HP Upgrades!
Won the Ice Skating Competition
All Shield Upgrades
Completed the Bros Quest
You gave Blobber his favorite drink
Made your first drink
First Shield Upgrade
First missing key used
Defeated The Moth Queen, Alexandria II
Your First Bribe
A tree full of beer
First weapon Upgrade!
Won the Ant's Gang Race
Killed 20 Enemies
First HP Upgrade!
First Music Cassette
Killed your First Enemy in Combat
Finish the Intro