The Guardian was a type of monster created by Morganna Mode Gone to serve as her servants in The World. Completely obedient to her will they were dispatched to quickly dispose of any threats, player, AI or otherwise that attempted to stand against her will.
Each Guardian appears as a gelatinous jelly like creature in the shape of a dumbbell. At the center of each creature is a Twilight Bracelet which gives the Guardian access to the powerful Data Drain skill. Unlike the data drain used by a Phase the Guardian's data drain seems to be quite weaker, causing only temporarily unconsciousness and short term memory loss in those hit by it. Even without this power a Guardian is a powerful opponent, with extremely high HP, max stats, and strong attack skills.
The Guardians first appeared in SIGN, where they were under the control of the Phase Maha. Maha frequently summoned Guardians to deal with people who posed a threat to either Tsukasa, or Morganna's plans, several characters such as Silver Knight and even Tsukasa himself would fall victim to it at some point or another during the series. As a gift to earn his trust Morganna bestowed Tsukasa with a special Guardian that was under his direct control, though Tsukasa was at the time the equivalent of an unawakened Epitaph Wielder, and couldn't properly control it until after special training.
At the end of the series, as Tsukasa grew closer to awakening Aura, Morganna sent several purple Guardians to attack him and the players close to him. Most notably the player Subaru. Maha, who had come to Tsukasa's side seized control of Tsukasa's Guardian and used it to attack the ones controlled by Morganna. Later Maha would appear again to help Tsukasa, this time sacrificing herself to the pursuing Guardians in order to save him.
At the end of Wotan's Spear Albireo attempts to attack a Guardian being controlled by Maha. To his surprise his attack is deflected, shattering his Spear of Wotan. As punishment for his insolence the Guardian Data Drains him, placing him into a temporary coma.
Kite encounters Guardians for the first time in .hack//Quarantine where they are guarding areas protected by Morganna. Later he accompanies Elk to the bottom of a dungeon where he finds Maha imprisoned inside of a Guardian. Destroying the Guardian the two free Maha, who transforms into Mia.