Surprisingly for a character created by comic superstars Grant Morrison and Mark Millar, Aztek's story is a pretty short one.
The boy known only as Uno was raised by the mysterious Q Society to be the champion of the Aztec god Quetzalcoatl and battle his rival god Tezcatlipoca (thankfully it was a fight, not a spelling competition). Donning a magic suit that gave him near limitless powers, he became Aztek 'The Ultimate Man' and moved to America, taking up the identity of the recently deceased Dr. Curt Falconer.
The Ultimate Man didn't really live up to his moniker though. He briefly joined the Justice League, but quit when he discovered one of the Q Society's benefactors was Lex Luthor. Then he discovered Tezcatlipoca was actually a world destroying machine call Mageddon and died fighting it. He wasn't even the one who stopped it; he died to give Superman a chance to beat his archenemy.
Ironically, the first issue of Aztek's series advertised him as "A hero for the new millennium... if he lives that long." And he did... just. Aztek died in May 2000 and hasn't been mentioned since