Bakura Ryou is one of Yugi's class mates. He is possessed by an evil spirit from the Millenium Ring. He is out to get all seven of the magical Millenium items but does not succed.
He is not evil for some of the anime but when he tries to steal Yugi's Millenium Puzzle the first time we learn of the evil in him from the Millenium Ring. Yugi ends up beating him and they send the evil Bakura to the Monster's Graveyard. Some how he comes back though and tries to steal Mokuba's body when his soul is trapped in one of Pegasus' cards.
