The American hero is a Nightmare Mode monster that spawns in the dungeon once the Keeper of Souls is defeated. It uses the American Shield to attack.
It looks like it has blue armor on, carrying a shield with red, white, and blue stripes with a silver star In the middle.
Attack: 50 (Contact) 125 (Shield)
Knockback Resistance: 100%
1 gold coin (100%)
American Shield (25%)
This is a reference to Captain America, the marvel superhero.