The strange dark spirit that lies inside the Millennium Ring. He's the main arch enemy of Yami Yugi. He holds a 3,000 year old grudge with Yami Yugi. Dark Bakura seeks the Millennium items to open the Door of Darkness, which grants evil power to anyone that opens it. To do so, he takes control over Bakura's body against Bakura's will, since he doesn't have a body of his own. In the beginning of the story, he torments Bakura by taking over his body whenever he played a game with his friends and used Penalty Games to trap their souls into TRPG miniatures for the Monster World game, causing Bakura to constantly transfer schools. With the help of Yugi and Yugi's other self, Jonouchi, Anzu, and Honda (and Miho in the 1998 anime), they were able to temporarily purge Dark Bakura's influence on their friend by defeating the final boss of Monster World, Dark Master Zorc.
However, later on when the group is seemingly going to be trapped within the labyrinth below Duelist Kingdom, the spirit's voice within the Millennium Ring tricks Bakura into putting it on again, assuring Bakura that his other half will help save his friends and that the dark spirit has undergone a change of heart. With Bakura's friends unaware that he had once again put on the Millennium Ring, Dark Bakura helps Yami Yugi in his game designed to outwit the Meikyuu Brothers' trickery and they head to the surface. For a while in the manga, it was unknown whether or not Dark Bakura was still a malevolent spirit. On one hand, he occasionally helped Yugi and his friends, trying to gain their uneasy trust and seemed to allow Bakura more free control of his body compared to his earlier treatment of his host (who was still unaware of what transpires whenever his other half took over). On the other hand, unbeknownst to the others, he is shown to be the one that killed Pegasus by tearing the Millennium Eye out of his eye socket and taking it for himself, licking the leftover blood. It's soon clear where his true standing was in the Dungeon Dice Monsters arc: after giving Yugi morale support during the game against Ryuji Otogi and helping him retrieve the pieces of the shattered Millennium Puzzle, Dark Bakura secretly plants a portion of his soul into one of the pieces to uncover the True Door from within, and intends to do anything he can to manipulate everything into place until the day comes where all of the Millennium Items have gathered together, in preparation for the Shadow Role-Playing Game / Shadow RPG, with the ultimate goal of opening the Door of Darkness and unleashing the darkness sealed within the Puzzle. Unbeknownst to Bakura, throughout the later portion of the manga, his other self will occasionally take over his body whenever he sees the chance of furthering his own selfish goals.
(Source: Wikipedia, Yugipedia)
