Agent Sands

Agent Sheldon Sands, CIA

Once Upon a Time in Mexico

Basic bio (via IMDB): Sands operates under his own system of morals, considering himself to be the one to "keep the balance" in society. He maintains a calm demeanor, but is clearly quite wily and dangerous, as he kills without second thought and with prodigious skill, making him something of an anti-hero. He has a fondness for obvious costumes and cheesy gadgets, such as a fake arm that he uses to hide the gun in his real hand, and a faux mustache and beard.

In the film, a drug lord named Barillo has hired an army general named Marquez to kill the President and help him overthrow the government. Sands hires El Mariachi to make sure this does not happen and Marquez never takes power. It is unknown if this was his assignment from the CIA or just a personal plot, but he uses his manipulative talents to orchestrate a complicated plan, using El's painful personal history to his advantage.