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