The Guardian is an exceptionally powerful warrior who guards a time portal even from Aku. He appears in Episode XXXII: Jack and the Travelling Creatures. He is voiced by Kevin Michael Richardson.
Learning of a time portal from an ancient water-faring beast, Samurai Jack, assisted by several mythical creatures, arrived in what appeared to be a gigantic graveyard, full of the corpses of many enormous machines. Traversing it, he came upon a large, greenish-blue light source with a tall, black-clad man standing before it. Addressing the figure, Jack learned that the light source was the time portal that he sought, but was denied entrance by the man, who revealed himself to be its Guardian. He explained to Jack that he had spent many eons guarding the time portal, and had defeated all who had attempted to best him. He also inadvertently revealed that the countless corpses dotting the landscape around the portal were the bodies of all the warriors whom he had slain. Only the one man who was prophesied to one day defeat him would be granted access to the portal, and Jack, it seemed, was not that man. Jack however, was insistent upon using the portal, and challenged the Guardian. Beginning with a brief sword fight, the Guardian gleefully disarmed Jack with minimal effort, reiterating his claim that Jack was not the prophesied warrior.
In response, Jack picked up a halberd-like weapon from one of the many fallen warriors, and re-engaged the Guardian, who drew two sai. After a short bout, the Guardian took Jack's weapon, broke it in two, and attempted to kill him with it. Jack managed to dodge all of the Guardian's strikes, and in response, the Guardian drew a machine gun with a rocket launcher and shot at Jack. Running for his life, Jack attempted to retaliate with a number of throwing stars scavenged from the battlefield, but the Guardian drew another gun and shot them out of the air.
Desperate, Jack managed to scrounge up two shield-like pieces of metal to block the Guardian's bullets, and kicked two of the Guardian's rockets back at him. Believing the battle to be won, Jack was astounded to discover that the rockets, despite having hit the Guardian head-on, had done nothing more than to ruin his suit. Enraged at the destruction of his favorite outfit, the Guardian charged Jack and proceeded to pummel him with his fists, destroying Jack's shields and repeatedly knocking him about.
Jack, clearly outmatched, expressed his astonishment at the Guardian's capabilities, and the Guardian once again repeated his assertion that Jack could not best him. He offered Jack the opportunity to leave, but Jack, determined to return to the past, charged the Guardian yet again. However, the Guardian was simply too powerful for Jack to harm, and he easily overwhelmed Jack, knocking him unconscious. The Guardian then prepared to crush Jack beneath a giant boulder, but the portal itself suddenly gave him a message that caused him to put the boulder down in realization about Jack and summon a pterodactyl-like creature to carry Jack to safety.
After Jack's departure, the Guardian, thinking aloud, apologized to Jack, saying he could not use the portal yet, and turned to the portal, where an image of a bearded, older Jack with a red cloak was displayed while standing on a black mound.
Within the next 50 years, Aku destroyed almost all the time portals he knew of, and Jack claims that all known time portals are destroyed as well. Given it is supposed to be Jack's destiny to use the one that the Guardian protects, it is possible that it and he both escaped Aku's notice, but no indication has been given either way yet.
In Episode C, it is implied he was killed by Aku during the 50 year time gap, as evident by his broken glasses and destroyed time portal.