The anime Yu-Gi-Oh! kept the same core cast of characters for its entire five-season run; Yugi, Joey, Téa, Tristan, and of course, Pharaoh Atem were part of every adventure. Joining them was a cast of supporting characters who at times were just as vital to the story, mainly Seto Kaiba and his obnoxious little brother, Mokuba.
Each season introduced new and exciting characters into Yugi Muto's journey. Some were friends, some foes, and some others just nuisances in the young boy's road. But they all contributed something to the story, adding extra layers of complexity into the already absorbing plot.
Odion
Odion is a tragic figure in the anime. He is Marik's adoptive brother and de-facto protector, who carries immense guilt for Marik's descent into madness and obscurity. He acts as Marik's advisor and confident, always willing to support him but never actually daring to defy or go against his will.
Unlike Yami Marik, Odion is honorable and valorous and has the spirit of a true duelist. He incurs the wrath of the Egyptian God, the Winged Dragon of Ra, and is struck down by lightning during his duel with Joey. He eventually recovers and goes back to Egypt with a redeemed Marik. Odion keeps appearing, but in a much smaller capacity.
Marik's Rare Hunters
For nearly half of the Battle City arc, Marik remains hidden, unwilling to reveal his true identity to Yugi and the Pharaoh. He uses his Millennium Rod to control a wide assortment of characters, each stranger than the previous one.
All these underlings get their moment to shine, despite appearing in only a couple of episodes each. Among them is Arkana, the deranged magician obsessesed with winning back his long-lost love, Strings, the quiet and punk-like young man who initially has Slifer the Sky Dragon, and Lumis and Umbra, the tag team that challenges Yugi and Kaiba to an unforgettable double battle.
Dartz
Dartz is the main antagonist in the Waking the Dragons arc. He is the ancient King of Atlantis, who fell under the negative influence of the Orichlacos and brought about his kingdom's ruin. After the Battle of Atlantis, he spends the next 10,000 years roaming the Earth, searching for human souls to reawaken the Great Leviathan.
Dartz is mostly a talky antagonist. He lacks Pegasus's panache and Marik's overwhelming menace. He holds a lot of power and is overall an effective villain for Yugi and company. So much more could've been done with him, though, and fans can't help but consider him something of a missed opportunity.
Ishizu Ishtar
Arriving at the beginning of the Battle City arc, Ishizu is Marik's older sister and the Millennium Necklace's guardian. The item allows her to see through time, offering vivid glances of things to come and past events. Ishizu uses it to foresee the events of the Battle City tournament, intending to awaken the Pharaoh's memories and stop Marik's plans.
Ishizu is a wise, kind, and spiritual woman who never loses her cool. She has more knowledge about the past than anyone else in the show but always speaks in riddles, needlessly complicating things. Ishizu is one of the Pharaoh's most valuable allies and a true Tomb Keeper.
Leon Von Schroeder
Leon is a talented duelist and of the few memorable characters from the Grand Championship arc. He runs a deck based on fairy tales and has monsters like Cinderella, Forest Wolf, and Little Red Riding Hood. Leon is polite, lively, and has great respect for Yugi as a duelist.
Leon turns out to be Zigfried's younger brother, who wants to help restore his family's reputation. Zigfried hijacks his final duel against Yugi, forcing him to use Golden Castle of Stromberg. Yugi manages to defeat the card, though, and Leon loses the duel but harbors no ill feelings towards Yugi.
Rafael
Unlike his two comrades, Valon and Alister, Rafael is actually an interesting and even necessary character in the Waking the Dragons arc. He is a formidable foe to Yugi, who even manages to defeat him in duel, briefly taking his King of Games title.
Rafael's backstory is a bit clichéd, and his motives don't make a lot of sense. Still, the character is compelling and intriguing enough to justify his presence, and his rivalry with Yugi is a highlight of the season as a whole. It would've been nice to see more of him, but he doesn't return save for a brief and silent cameo near the series' end.
Mana
Mana plays a pivotal role in the Pharaoh's Memories arc of the series. She is Atem's childhood friend and Mahad's student who's also a capable magician in training. She bears a strong resemblance to Dark Magician Girl, a monster that eventually becomes her own Ka.
Mana is a playful, enthusiastic, wholesome, and caring girl who still displays a great deal of courage and determination. She plays an important role in the final battle against Zorc and is also among those welcoming the Pharaoh into the afterlife.
Priest Seto
Priest Seto is one of the six Sacred Guardians who serve Pharaoh Atem. He shares several traits with his present-day counterpart, mainly his cunning and ruthlessness. However, unlike Seto Kaiba, Priest Seto is kinder, more empathetic, less selfish, and more open-minded.
Priest Seto is also a valuable ally and confidant of Atem. He also shares a bond with the Blue-Eyes White Dragon and holds control over the Millennium Rod. Priest Seto gets his own side story during the Pharaoh's Memories arc, which involves his father and a mysterious blue-haired woman named Kisara.
Bandit King Bakura
Despite being a secondary antagonist throughout the show, Bakura Ryou takes center stage until the last arc of the series, as does the original entity of Yami Bakura, Bandit King Bakura. Also known as Bakura, King of Thieves, he is originally an inhabitant of the village of Kul Elna. He witnesses his people get massacred to create the Millennium Items, becoming enraged and resentful. His own hatred, mixed with the vengeful spirits of Kul Elna, allows him to create his powerful Ka, Diabound.
Bandit King Bakura is easily the series' second-best antagonist. He is powerful, resourceful, entertaining, and at times even funny. He is a true agent of chaos and comes closer than anyone else to bring Yugi and the Pharaoh to their knees.
Marik Ishtar
Marik is the best antagonist of the show, without a doubt. His shadow looms large over the entire franchise, a testament to what a great villain he is. Marik is a Tomb Keeper who becomes resentful and envious of the Pharaoh's power and begins a crusade to take it for himself.
During the Battle City arc, Marik attempts to steal the Millennium Puzzle numerous times but always fails. He also becomes somewhat of a comedic figure during the Enter the Shadow Realm arc, where he stays aboard the Kaiba plane while everyone else becomes trapped in Noah's virtual world. Marik eventually redeems himself and becomes an ally of Yugi and the Pharaoh, but fans will always remember him as the villain in what is, arguably, the show's best season.