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