Obelisk the Tormentor

Obelisk the Tormentor is the first of the cards to appear in the anime. Obelisk is the only card Marik could not acquire, and his sister Ishizu gave the card to Seto Kaiba in the hopes he would use the card in his upcoming tournament and lure Marik out of hiding to claim it for himself. Kaiba used Obelisk as his trump card during the preliminaries, but lost to Yugi Mutou in the semi-finals and thus handed the card to him. Yugi went on to use Obelisk against Marik in the tournament finals.
Obelisk makes sporadic appearances throughout the rest of the series. It is used by Gurimo in Season 4 of the anime, and takes part in the battle against the Leviathan. It is used to battle Bakura's Diabound in the final season; it defeats Diabound in the manga, while it is a draw in the anime against the power of the Blue-Eyes White Dragon that Diabound had absorbed. In the manga, it is the only God card that Atem uses in his duel with Yugi (in the anime, he uses all three Gods).
Obelisk's ATK and DEF are 4000 and its effect allows a player to offer two monsters as Tribute to destroy all of an opponent's monsters and inflict 4000 damage to the opponent. When used by Seto Kaiba, Obelisk was used multiple times to achieve a one-turn kill victory in such a manner. It is also shown that Obelisk is able to achieve an infinite attack strength. However, this only happened in Yu-Gi-Oh! The Movie: Pyramid of Light, and a single time in the canon series which was then explained as "a miracle of Obelisk's anger" and not confirmed as an actual effect

Blue eye White Dragon & Dark Magician Girl

The signature card of Seto Kaiba, it is considered to be one of the strongest monsters in the anime. Its attack is White Lightning in the English anime, and Burst Stream of Destruction in the Japanese anime. In the manga and anime, there are only four in the entire world, the explanation being that the card was considered "too powerful" and therefore discontinued. Kaiba, who dreams of owning all of them, acquires three of them through underhanded tactics such as mafia deals, or forcing the owners into bankruptcy. Only three of one card can be in one's deck at a time, and the fourth Blue-Eyes belonged to Sugoroku Mutou (Solomon Muto in the English anime). After claiming the final Blue-Eyes, Kaiba rips it up so it can never be used against him.
The card becomes Kaiba's trademark card and a motif of sorts, as he owns a jet, a dueling dome and a theme park all based on the beast. It is rumored that the card Kaibaman was created by Kaiba himself and is heavily based on him in appearance. In the Yu-Gi-Oh! GX episode in which Kaibaman appears, he even has the same voice, deck and mannerisms. Kaiba's appearance of a white coat and bright blue eyes also conjure the dragon's image. It should also be noted that Kaiba's armor in the PlayStation 2 video game Duelist of the Roses is themed from the Blue-Eyes White Dragon as well.
In the final season of the anime, the dragon is revealed to be the spirit of Kisara, a young woman who was captured by Akhenaden for the power of the beast. Kaiba's past incarnation, Priest Seto, fell in love with Kisara and defended her to death. Priest Seto first became interested in the Blue-Eyes when he saw Bakura's Diabound use Burst Stream to defeat Obelisk the Tormentor - he cites this incident as indicating there exist monsters with powers that surpass the Pharaoh's God Cards. This leads to his search for stronger monsters that eventually leads him to Kisara. After Akhenaden killed her and possessed Seto, Kisara's spirit entered Seto's mind and destroyed Akhenaden. The love the two shared is the reason why Kaiba was driven to claim all the Blue-Eyes cards in the present.
In the anime, the monster itself is described by Zorc as "the ultimate creature of light," with power greater than that of the three God Cards. The second Yu-Gi-Oh! Movie supports this by having Blue-Eyes and its forms representing Kaiba, the King of Light, and being his strongest creatures. In the anime, the Blue-Eyes White Dragon's power is further demonstrated by being the first monster who deals actual damage to Zorc, cutting off Zorc's forearm with a blast (although Zorc still beat Blue-Eyes). Even Exodia, considered by many to be invincible, was unable to do this: while he did damage Zorc, Zorc was able to regenerate.
The Blue Eyes Ultimate Dragon is portrayed in the anime as an evolution of the Blue-Eyes White Dragon. It was capable of deflecting all of Zorc's attacks with its own Triple Burst Stream. The Blue-Eyes Shining Dragon and Dragon Master Knight (known in the game as Master of Dragon Soldier) are further evolutions of the Ultimate Dragon. Blue-Eyes Shining Dragon is summoned by first sacrificing a Blue-Eyes Ultimate Dragon. It was only featured in the second movie. Dragon Master Knight, a combination of Blue-Eyes Ultimate Dragon and Black Luster Soldier, made a few appearances throughout the anime. Each time it is summoned through a combined effort of Yugi and Kaiba. In the TCG the Blue Eyes Ultimate Dragon is commonly used for One-turn kill deck types.

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A female counterpart to the Dark Magician, she is another of Yugi's trademark cards. Following her introduction into the anime, she gained a greater deal of popularity among the fandom. She, unlike the Dark Magician, has appeared as a Duel Spirit twice.
The Dark Magician Girl played an important part in the fourth season of the anime, she was the representative of the Duel Monsters' world who begged Yugi and Yami Yugi to stop Dartz. She would eventually lead the forces of her world in an all-out assault on the Leviathan in an attempt to destroy it, but this failed. In the final season of the anime, the card is revealed to be the Ka Monster of Mana, the Pharaoh's childhood friend and Mahad's pupil/successor.
Her Duel Spirit appeared once again in Yu-Gi-Oh! GX. During a school festival, the one day Duel Spirits can take physical form, she appeared to duel Jaden Yuki. The crowd cheered actively for her, and even though she lost, she didn't care because she had so much fun. Before the day ended, she used the last of her energy to give Syrus Truesdale, who she thought was cute and who also had a crush on her, a kiss on the cheek. She plays with a Spellcaster deck, which centers around her own card.