Here is the full list of all 44 Underhero achievements.
All weapon Upgrades!
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!
Bribed 20 Enemies
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
All Shield Upgrades
Defeated El Salserissimo
Cook the spiciest salsa
Found the Hidden Treasure of the beach
Complete Jilani'snPepper Collection
Half Music Cassettes
Cleared the Pica Pica Coliseum
Half HP Upgrades!
First Shield Upgrade
Defeated Baron van Gregory
Defeated The Grey Kid
Won the Ice Skating Competition
Half weapon Upgrades!
You gave Blobber his favorite drink
Completed the Bros Quest
Made your first drink
Your First Bribe
Defeated The Moth Queen, Alexandria II
First missing key used
A tree full of beer
First weapon Upgrade!
Killed 20 Enemies
Won the Ant's Gang Race
First HP Upgrade!
First Music Cassette
Killed your First Enemy in Combat
Finish the Intro