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