Here is the full list of all 44 Underhero achievements.
You deserve a raise
Spared the Masked Children in Heroes Road
Read all Robot Text Logs
Defeated ???
The End of The World
Defeated Mr. Stitches
All HP Upgrades!
All Music Cassettes
All weapon Upgrades!
Recruited The Baron
Recruited the Moth Queen
All missing keys used
Eased a Fathers Mind
Defeated T. Bur
The Show must go on
Night Theater
All Shield Upgrades
Defeated El Salserissimo
Cook the spiciest salsa
Bribed 20 Enemies
Found the Hidden Treasure of the beach
First Shield Upgrade
Complete Jilani'snPepper Collection
Half Music Cassettes
Cleared the Pica Pica Coliseum
Half HP Upgrades!
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
Your First Bribe
First missing key used
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 Music Cassette
Killed your First Enemy in Combat
Finish the Intro