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When Seto was 3, his mother died giving birth to his young brother Mokuba. Five years later, their father died in a car accident, and Seto vowed to take care of his brother and give him a normal childhood he himself never had. They were sent around for two years to their greedy relatives who took all of their inheritance and left the two at the orphanage. Seto showed genius-like intellect at a young age but despite the number of parents that wanted to adopt him, he refused to leave without his brother whom he was a guardian to.
On the outside, Seto Kaiba has an intimidating reputation. Not only was he the World Champion of Duel Monsters (until he was defeated by Yugi), he is also the president and CEO of the world's largest gaming company, Kaiba Corporation - all at the age of 16 (18 in dub). He is known to be ruthless in business and battle, and treats Yugi and his gang with contempt, often giving them sarcastic and derisive remarks. However, he later comes to respect Yugi (and arguably only Yugi) greatly for his dueling ability. Kaiba is driven by his passion for Duel Monsters and his drive to achieve his own goals and owns all three of the only Blue Eyes White Dragons in the world. He tore the fourth one that belonged to Yugi's grandfather because he didn't want it to be used against him.
Seto realized his lucky break when Gozaburo Kaiba, world chess champion and owner of Kaiba Corporation (a weapon making company), decided to visit the orphanage for a publicity stunt. Seto challenged Gozaburo to a chess match, saying that if Seto won, Gozaburo would have to adopt both brothers. Gozaburo agreed and lost. Devastated but impressed by Seto's preciousness, he worked overwhelmed 10-year-old Seto with advanced classwork and tortured him to the point of arguable mental abuse. He also prevented Mokuba from seeing Seto, which was perhaps the biggest devastation. Gozaburo told Seto on his 13th birthday, that he would have 10 million yen for one year, but that he must multiply the amount by 100 by his 14th birthday if he did not want to return to the orphanage. With his immensely calculative mind, Seto managed to do it in a day by buying a small company and forcing the owner to repurchase it. Gozaburo was impressed once more, but no less harsh on his adoptive son. He began a game with Seto, in which Kaiba Corporation's controller rights were at stake. Seto managed to outwit Gozaburo and gained control of 51% of Kaiba Corp's stock shares, earning him the title of CEO. Gozaburo, humiliated and devastated, disappeared, leaving the now hardened and cold Seto Kaiba in control. He rebuilt KC from the ground up, making it into a gaming technology company where he created most the technology related to Duel Monsters, also bringing him one step closer to his dream of creating Kaiba Land, a duel monster theme park. For the next period of time until he was devastatingly defeated by Yugi, Kaiba was feared and respected, unreachable by all except his little brother Mokuba, whom he has always had a protective streak and soft spot for. His attitude towards Mokuba (who respects his brother greatly) contrasts with his attitude towards the rest of the world he had come to resent. Kaiba lives by his rule to never look back at the past and never lets anything stop him from conquering the future. While his antics are rather extreme and manipulative, he is a man of his word.
It is later revealed in the Duel Kingdom arc that Kaiba was once business partners with Maximillion Pegasus. Pegasus "created" the game Duel Monsters (after he heard of its existence in ancient Egypt) and Kaiba created the holograms to bring the monsters to life. However, Kaiba later finds out that his executive board (five left over from his father's term) had betrayed him to join with Pegasus who wanted to take control of the company. Pegasus kidnaps Mokuba while Yugi and the team are at Duelist Kingdom. Before Yugi enters the castle with his star chips, Kaiba prevents him from entering, saying he must duel him because he also needs to enter the castle to rescue Mokuba (who was locked in a dungeon). After Yugi is about to win (fair and square), Kaiba goes to the farthest lengths to secure victory by stepping on the edge of the castle wall. Yugi calls off the last attack, afraid the shock waves would make Kaiba lose his balance. Kaiba impassively clutches the chips and enters the castle to duel Pegasus. However, Pegasus wins by using his Millennium Eye to read Kaiba's mind, thus imprisoning both Seto and Mokuba's souls in Duel Monster cards. It is Yugi who wins in the end to free them all, along with his own grandpa.
In Battle City, Kaiba hosts a tournament in efforts to reprove his skills and defeat Yugi. He is given Obelisk the Tormentor by Ishizu Ishtar (who wished to keep the card away from her brother Marik - the leader of the rare hunters). The tournament ends up with several near casualties, but Kaiba states that those who cannot handle it should go home. Right as the KaibaCorp blimp is en route to KaibaCorpo Island where the final duel would take place, the blimp suddenly is hijacked by a mysterious boy named Noah. The blimp docks inside the fortress and the group is met by some old foes - Kaiba Corp's former Board of Directors, The Big Five - who inform them that they are now part of Noah's Virtual World. Kaiba and Yugi's gang are forced into duels against each of the Big Five (whose minds had been trapped in the virtual world and could not escape unless they took over other bodies).
It is revealed that Noah is actually Gozaburo Kaiba's real son, who died in a car wreck at the age of 10. Gozaburo used his advanced technology to save Noah's mind in the digital world until he could find a body for him. That was the original plan he had for Seto's body, until he realized that Seto's mind was perfect for his plans. Noah was forced to stay in the virtual loneliness and came to resent Seto. Noah utilizes various underhanded tactics to lure Seto into battle in which he hoped to defeat him to show his father he was better than Seto. After turning Mokuba against Kaiba by brainwashing him, Kaiba was forced to battle with Noah who had invincible monsters at his disposal. Noah won by cheating and turned both the Kaiba brothers into stone right before they reached each other after their separation. Yugi, furious and determined to stop Noah, told Noah he would finish Kaiba's duel. Noah turned each and every one of Yugi's friends into stone to mess with Yugi's head, but Yugi still managed to pull tricks out of his sleeves to ensure the victory. Before the finally finishing blow, Yugi called Noah out for being a weak, insecure child, also stating that Seto was far more of a duelist and person than he could ever hope to be. Realizing his mistakes, that his father still disrespected him, and seeing how Mokuba still treated him like a brother even after all he had done, Noah felt extreme guilt over his actions. He ends up sacrificing himself by wiping away both his and his father's memory in the virtual world. However, Mokuba states in the ending scene that knowing Noah, he probably saved his mind in a back up file. The Battle City tournament then commences at the tower.
Seto Kaiba has also has the powerful monster Chaos Emperor Dragon - Envoy of the End. This card was used against Ziegfried von Schroeder and his Valkyries. Chaos Emperor Dragon proved to be a powerful entity, capable of annihilating the whole field along with the hand, although unfortunatly destroyed itself in the blast. Chaos Emperor Dragon, like all other Duel Monsters, must've had been a Ka at one point in order for Pegasus to design it as he did the Egyptian Gods and the other cards. As a Ka, going off of his card effect, Chaos Emperor Dragon was most likely able to destroy a country with ease, and possibly a continent.