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