Dark Marik, the twisted split personality who drove the real Marik off the deep end in the first place. Seizing control of Marik's body during his childhood, he killed their Abusive Dad, then tried to kill their adoptive brother, who had been keeping Dark Marik locked up. This failed, and Dark Marik bided his time, gaining strength, and driving Marik to further and further acts of cruelty in his quest to Take Over the Worldnote including, but not limited to, kidnapping, Mind Rape, brainwashing, and attempted murder. Resurfacing when Marik's brother was defeated in the Battle City semifinals, Dark Marik exiled his better half from their shared body, then engaged Mai Valentine, The Spirit of the Millenium Ring, and Joey Wheeler in a series of increasingly sadistic Shadowgamesnote Mai lost memories whenever a monster was destroyed, the Spirit of the Ring lost parts of his body, and Joey lost life energy that saw all three damned to the Shadowrealm. He also made several attempts on the lives of his sister and comatose adoptive brother. This all culminated in a final battle against Yami, during which he arranged for any damage the two of them incurred to be suffered by their better halves, Marik and Yugi, nearly killing them both. An Axe CrazyCombat Sadomasochist with omnicidal designs on the world, Dark Marik left the cast in shellshock and irrevocably altered the tone of the show.
The Spirit of theMillenium Ring was the show's longest running antagonist, and one of its worst, possessingYugi's friend Bakura for almost the entire duration of the series. During the Duelist Kingdom arc he traps Yugi and his friends within the card game, resulting in their being sent the graveyard if defeated. He subsequently tries to bodyjack Mokuba Kaiba, and rips Pegasus' Millenium Eye out of his eye socket, leaving him in a coma. During Battle City, he infiltrates the tournament and sends Bonz, Zygor, and Sid to the Shadowrealm, then tries to do the same to Yugi and Yami. Failing, he returns in Season 5 as The Big Bad. Outing himself as an agent of Zorc Necrophades, The Spirit of the Ring repossesses Bakura, and sets in motion a plot to bring about The End of the World as We Know It. He kidnaps Mokuba to force Kaiba into a Shadowgame, then traps the entire cast in the Memory World, where he can control their lives, possessing Tristan and his own past self, Thief King Bakura, in the process. Eventually pulling a You Have Outlived Your Usefulness on his own past self, The Spirit of the Ring unleashes Zorc in the past, while taking on Yami in his final Shadowgame in the present, with the intent of making the entire world bow down to his dark god.
Brron, Mad King of the Dark World, is demented and sadistic, ruling the Spirit World as its vile dictator and being responsible for its deplorable state and oppression of its denizens. His right-hand man, Zure, oversaw Brron's will, detaining anyone who defied him in internment camps while killing duelists who could be threats to him. The executions occurred at his fortress and involved releasing monsters at disarmed duelists while Brron himself and his minions watched the spectacle. After learning of Jaden's presence in his domain, Brron orchestrated a plot that involved turning Jaden evil by sacrificing all of his friends one by one with his set of "Wicked Doctrine" cards to create the "Super Fusion" card. After his defeat, Brron mocked Jaden by saying that he would never see his friends again before dying.
Divne/Sayer is the leader of a group of duelists training to control and harness their psychic abilities in order to turn them into soldiers of war against the rest of humanity. His earliest duelist, Toby Tredwell, was unable to withstand the procedure meant to test his abilities. Believing him to be weak and useless, Sayer ordered the intensity of the electric shocks to be amplified, which caused the young boy's death. He also does his best to keep his prized duelist Akiza secluded from anyone outside the Arcadia Movement to keep her full of hate and resentment at the world for his goals, as well as hide his own secret motives. To this end he gasses Luna, Leo, Yanagi and Bolt Tanner when they come to him for aid against the Dark Singers. Then he threatens Leo's life in a psychic duel to test if he has powers like his twin sister Luna. Later that day he has another Psychic duel with and murders Carly Carmine by sending her through a window with a direct attack, kick-starting her rebirth into a Dark Signer. He then returns from his Disney Death at the hands of Carly, posing as a security agent to attack Yusei and Mina. Sayer states that anybody who falls prey to him will simply follow his orders and nothing more, as he takes full advantage of that individual's weakness, just like he did with Akiza.
Lotten/Lawton is a tyrant lording over a small western style town called Crash Town, who forces those he's defeated to join him or face slavery in the mines. He gets into a match with Yusei while Yusei and the gang are trying to escape the mines, but before it can be decided, he throws a stick of dynamite at Yusei, Kalin, West and Nico that results in cave-in. When it explodes, Yusei and Kalin fall off the mountain, while Nico and West are unconscious on the ground. Lawton takes them back to Crash Town, renaming it Lawton Town after Barb and him double cross his brother Malcolm. Later, not satisfied with the number of workers in the mines, he and Barb begin forcing their henchmen to Duel for their survival, which is stopped by Yusei and Kalin. Once he does start to lose his 2-on-1 handicap match against Yusei and Kalin, he quits the duel and tries to run away, but not before he abandons Barb to her fate and set off explosives in the town.
Barb was just as bad, and in many ways, even worse, seeing as she used seduction and deceit (and it was hinted, sexual favors in Malcolm's case) to betray anyone who stood in their path. After even Yusei fell for it, she offered to spare him from the mines if he was willing to become her personal slave; his rather blunt refusal caused her to strike him. Ironically, even though Barb never dueled once in the story (whether she even did that or not isn't known) she remains one of the most reviled villains in the franchise among fans.
Vector is an Emperor from the Barian World. He manipulated Dr. Faker and Vetrix to fulfill his own agenda of destroying the Astral World. After his first defeat at the hands of Yuma Tsukumo, Reginald Kastle and Kite Tenjo, he orchestrated a new scheme by taking on a human form on Earth and calling himself Rei Shingetsu (Ray Shadows in the dub), becoming friends with Yuma as a gambit to orchestrate the latter's fall. When that failed, Vector released the god of the Barian World, Don Thousand, and became his host. Vector also betrayed and murdered his fellow Barian Emperors, Nasch and Merag in their past lives, and in the present, he's so abusive that even the other six can't stand him.
And he finally tops himself in this regard by double crossing them again after absorbing Number 96 and becoming even crazier than before.