American Crusader is a homage/doppelganger character based on Captain America. He appeared first time a in Final Crisis: Superman Beyond 1 and had a developted story in The Multiversity. Along with his companions, he is part of the Retaliators, a homage team of Marvel Comics' Avengers characters.
Native of the universe know as earth 8, American Crusader is the leader of the Retaliators and he represents the spirit of his team: Ready.
When the heroes from several parallels universes crashed in his homeworld, the American Crusader was the first one to stand against them, being cautious about their origins and intentions but he was ready to stop the attack of interdimensional forces upon his world, including the zombie versions of himself and his allies.
However the repeated multiversal shenanigans made him and the heroes of earth 8 to see the other worlds suspiciously. When Tartarus attacked his world to reack the multiverse crack than could reveal where was Barry Allen, Crusader fell to the forces of his enemy. His shield now is weild by a new Crusader, who could be his friend Silver Eagle. That is, until his eventual resurrection.
Although American Crusader is not the first Captain America inspired homage character, (being preceded by Freedom Fighters' Americommando) he has been the more stable. Also, there are others variants of the character.
On Earth 7, an alternate version of the character know as Crusader, died along with his earth and returned to stalk his earth 8 alternate.
On earth 10, The American Crusader is a comic book character very similar to the Nedor's American Crusader, but used a shield..
As the Marvel character, American Crusader also uses a shield as main weapon. At difference of Captain America, American Crusader have a shield capable of expanding itself to protect him and others from any attack or danger.