Marik is the brother of Ishizu. Unlike his sister Ishizu he is not stable meaning he has a split personality, or Personality disorder. He is the member of the tomb guardians, or the people who guard the Pharaoh's memories. His other personality is sealed in the millennium rod. He wants revenge on Yami because Shadi told him that the Pharaoh killed his father when it was his other personality. Then that drove him off the deep end. He formed the rare card collectors called the ghouls. They are spread out all across the world and pick up information. This is how they found out that Pegasus had made God Cards. They then headed to Egypt where Pegasus had buried the cards and dug them up. However Ishizu stole Obelisk and then gave it to Seto. He found out and let her go. His dark side is sealed by Odion's scars and that keeps him sane. The veins on his face tell you that he is under a lot of stress when he transforms. He gives Slifer to one of his best Ghouls who lose to Yami. He has the power from the rod that lets him enter peoples minds and take over their bodies. Then he has the Ghouls win all 7 puzzle pieces for him and he goes to the stadium. His dark side completely takes over when Odion is struck by lighting. Then his dark side faces Mai in which he wins and sends her in the Shadow Realm, a place where the darkness eats at your inner conscious. He then has a Shadow Duel with Bakura and barely wins by which he shows Ra's special power by fusing with it. He then takes the Millennium Ring and heads to the Final 4. He then faces in Katsuya in a dangerously close duel. He then faces Yami in a duel where if you lose your other personality is killed. Yami wins and switches His dark personality with his good personality and Marik is extremely grateful. Joey vs Yugi
This is after Marik uses two of his goons to face Yugi and Seto in a double duel. Marik got irritated on how Yugi would beat him time after time. So he kidnapped Joey and used his Millennium rod to take control of Joey. But Joey's sister was coming home from her eye operation and wanted to see him. Marik had told Yugi to come to the docks to face him and get Joey back. But they didn't know he was under his control. So he when they got to the docks, they trapped Tea in shackles challenged Yugi to a duel with his best friend. The rules of the duel were simple. The duelist would be put in shackles on their ankles and the duelist who lost would have an anchor dragged to the sea for the loser to drown. But their was a time limit, in 30 minutes they had to finish the duel or both of them would be dragged down to the sea.
So they were forced to face each other under Marik's control, but Yugi broke Marik's spell on Joey and set him free with the power of friendship. After that Marik had drowned joey into the sea and his sister saved him. And Marik had lost that round. Yugi vs Bakura This was the first duel of the semi-finals, after Bakura defeats three duelist for the 6 locater cards. He must face yugi "one on one". He is trying to get the millennium rod and puzzle. So he duels against yugi and starts to use these "FINAL" trap and spell cards. He almost brings all 5 to destroy Yugi, but Yugi summons the wind dragon of raw and defeats him. Marik vs Mia This was held with the semi-finals with Yugi, Marik, Seto, Mia, joey, Ishizu, Bakura, and Odiin. This duel was the second duel of the semi-finals. during this duel Mia had plans of summoning the wind dragon of raw from Marik, but you had to be able to read the ancient text on the card to summon it. Mia was only able to get raw out in his half form, but when marik said the ancient words it came to his side of the field and desroied Mia, then sent her to the shadow realm. Seto vs Ishizu This is the third round in the the semi-finals of the tournament. It is Marik's sister Ishizu vs Yugi's rival Seto kaiba. This starts off when Ishizu uses her Millennium Necklace to predict all of Kaiba's moves. Seto thinks that she is cheating and she is getting lucky, but everyone else doesn't. This duel is carried on to Seto almost about to be beat. But Kaiba finds a way to overpower Ishizu's power and summons "Obelisk the Tormentor" to defeat Ishizu. Joey vs Odion This is the last duel of the semi-finals. It was Odion vs Joey Wheeler. Joey and Odion had a good duel, but Odion was set for destruction. He tried to summon the copy of the wind dragon of raw but the power overwhelmed him and Marik's dark self had took over. Since Odion's energy and soul was sent to the shadow realm he couldn't continue the duel. As soon as Odion's soul left earth, Marik's dark self appeared and had made a big threat to Yugi. Joey vs Marik This was the first duel of the finals and it was tough. Joey had been losing to Marik most of the duel but joey was still fighting. On the last turn after Marik had used raw, joey had still been alive, He needed to summon and attack with his Gearfreid the iron knight and defeat him since Marik was open for attack. But the darkness overwhelmed joey and he couldn't defeat Marik, so he was sent to the shadow realm.