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