Don Krieg, also known as the "Pirate Fleet Admiral" and infamously nicknamed "Foul-Play Krieg," is a major antagonist from the East Blue. He hails from the East Blue and serves as the captain of the Krieg Pirates, a massive armada that once boasted a fleet of fifty ships and a crew of five thousand men. His name, Krieg, is the German word for "war," which reflects his aggressive and militaristic nature.

Krieg's personality is defined by cruelty, dishonesty, and an overwhelming arrogance. He is utterly convinced of his own superiority, believing that his immense military might and arsenal of weapons can conquer any foe. He lives up to his "Foul-Play" moniker by believing that victory is the only thing that matters in a fight, and he will employ any underhanded tactic to achieve it. He has no qualms about using deception, such as flying a white flag of surrender or disguising his ship with a Navy flag to ambush unsuspecting victims. Krieg views concepts like honor, pride, and mercy as pathetic weaknesses, and he is willing to kill his own subordinates for displaying such emotions. He rules his crew through fear and demands absolute obedience, often violently punishing any sign of hesitation or dissent.

Before the events of the main story, Krieg built his fearsome reputation by terrorizing the East Blue. His first act of piracy was hijacking a Navy ship, and he continued his rampage by attacking villages and passenger ships under false flags. Believing his armada was strong enough to conquer the Grand Line, he set sail for the more dangerous sea. However, only seven days into his journey, his entire fleet was decimated by Dracule Mihawk, one of the Seven Warlords of the Sea, who attacked out of boredom. Only Krieg's flagship, the Dreadnought Sabre, managed to escape the massacre, thanks to a convenient storm, and limped back to the East Blue with Krieg and a mere one hundred starving survivors.

Krieg's role in the story is as the primary antagonist of the Baratie Arc. He and his starving crew come to the floating restaurant Baratie, where Krieg initially feigns weakness and begs for food. However, once he has regained his strength, he reveals his true intentions: to steal the Baratie to serve as his new flagship, force its head chef to hand over his logbook of the Grand Line, and rebuild his forces to make another attempt at conquering the Grand Line.

His key relationships are primarily defined by fear and utility. He has no genuine attachment to his crew, viewing them as disposable tools. This is best exemplified by his relationship with his loyal first mate, Gin. Despite Gin's fierce loyalty and unwavering obedience, Krieg does not hesitate to order him to throw away his gas mask and die from poison gas as punishment for showing mercy to Sanji, the chef who had fed them. Conversely, his relationship with his rival, Dracule Mihawk, is one of utter inferiority. While Krieg is filled with rage at his defeat, Mihawk barely considers him a threat and only attacked his fleet to pass the time.

Krieg shows little development throughout his arc, serving as a foil to the series' themes of dreams and honor. His obsession with victory by any means necessary stands in stark contrast to Monkey D. Luffy's straightforward and honorable fighting style. After his defeat by Luffy, a delirious Krieg refuses to acknowledge his loss and is ultimately knocked unconscious and carried away by Gin, who vows that they will start over. Years later, Krieg reappears in the Final Saga, having somehow made his way to the pirate island of Hachinosu. His appearance has slightly changed, as he now wears glasses and seems to have aged, but he is still seen alongside his loyal subordinate Gin, suggesting that the Krieg Pirates have managed to survive and continue their journey.

Don Krieg is a Devil Fruit user, and his fighting style relies almost entirely on the vast array of weapons hidden within his massive, golden Wootz steel battle armor. His body is extremely strong, allowing him to wield his one-ton Mighty Battle Spear, which explodes on impact, with ease. His armor is equipped with numerous hidden firearms, including a machine gun that fires sharp, armor-piercing spikes, and a flamethrower. For defense, he possesses a spiked Porcupine Cape and uses diamond-studded gauntlets to augment his punches. His most devastating weapon is the MH5, a deadly poison gas bomb that he can launch from his shield, which is fatal to anyone who breathes it without a gas mask, a precaution Krieg himself always takes. Despite his vast arsenal, his primary weakness is his overconfidence in his weapons and his temper, which causes him to lose composure when his plans fail.