Yami Bakura
While on a trip in Egypt, Bakura's father purchased an ornamental ring from a street peddler, which he gave to his son because - according to the animated series - it was connected with the Duel Monsters card game, which Bakura was fond of. In truth, this object was the Millennium Ring, one of the seven mystically-empowered Millennium Items. Contained within the Millennium Ring was an evil spirit, which took possession of Bakura, and came to be known as Yami Bakura. Unlike the similar relationship between Yugi Mutou and the spirit of his Millennium Puzzle, the spirit of the ring is a malevolent entity, and completely suppresses Bakura's soul when it takes over his body.
The actual identity of the Spirit of the Ring is Zorc. The identity is revealed in Yu-Gi-Oh! the first series, Season 5, Episode 46.
Read more about this topic: Ryo Bakura