Bakura Ryou
Bakura is one of the badguys in Yu-Gi-Oh and in my opinion the coolest. He has been in all three seasons (two in America), but has not been the main badguy in any of them. Like Yugi, Bakura also owns a Millenium Item, the Millenium Ring. The Ring has the spirit of a tombrobber trapped inside and takes over Bakura Ryou's body. While in control he does many evil things. In America you first see Bakura during Duelist Kingdom tournament. He traps the souls of Yugi, Tea, Tristan, and Joey into their favorite cards and duels Yami Yugi. Bakura's own soul (the good one) is inside his favorite card, Change of Heart. When Evil Bakura tries to take over the Dark Magician which has Yugi's soul, the good Bakura stops him and takes over the Evil Bakura's Lady of Faith instead allowing Yugi to attack him. Yami Yugi though switches the souls of Evil Bakura and Good Bakura so the Evil one is sent to the graveyard. After the duel everything seems fine until the Evil one returns from the Shadow Realm somehow.
Biding his time so he can get all the Millenium Items, Bakura waits for Yugi to defeat Pegasus in a duel. After the duel Bakura confronts Pegasus in his sanctuary and reads his soul. They then proceed to duel and Bakura defeats him since Pegasus is still tired from dueling Yugi. Killing him, Bakura pulls out the Millenium Eye and licks the blood off of it. After stealing the Eye, Shadi comes to investigate the theft. Bakura pretends to be the Good Bakura to thwart suspicion from him. He doesn't do anything major until Bandit Keith comes in controlled by Malik. Malik uses Keith to steal the Millenium Puzzle but Bakura wants it for himself so he frees Bandit Keith's soul from Malik. Malik gets pissed and while he still controls Keith breaks the Millenium Puzzle. Yugi jumps out of the dueling ring to put the puzzle together with Bakura's help. Bakura secretly puts some of his own power into a piece of the puzzle, so he can always watch over it. He then watches secretly as the events of Battle City unfold.