Seto Kaiba is the antihero of the Yu-Gi-Oh TV series. He is Yugi Mutou's arch-rival and the CEO of the KaibaCorp gaming company. His primary goal is to be recognized as the greatest Duel Monsters player in the world.
As a child, Seto was a bright boy, protective of his brother, and mainly selfless. After he and his brother were adopted by Gozaboro Kaiba, he was forced to undergo vicious studying and and learning habits to allow him to understand how business works at its best. On Seto's birthday, he was given a 2% share of KaibaCorp (in the 4Kids dub, $10,000,000) and was told that although he could spend the money any way he wished, he must return 100 times (in the English anime, 10 times) the amount in one year. Seto returned all the money within a single day. He did this by buying 51% shares of a small company that cares for its employees, Seto then made the president buy it back, under threat of it being shut down and leaving the employees jobless.
Kaiba will stop at nothing to get his way, going so far as to drive his stepfather to suicide so he could inherit Kaiba Corp. He is also seen as a master of all games, but his favorite is duel monsters. Kaiba proves to be an incredibly greedy and sadistic monster with a sociopathic and borderline insane personality; having found out that Yugi's grandfather had the Blue-Eyes White Dragon card, the two dueled for the rare card, of which there are only four. As it turns out, Kaiba already has the other three of them and does not even need a fourth card. When Kaiba wins with these three, he realizes this and tears his prize to pieces, breaking Sugoroku Muto's heart. Dark Yugi takes over and duels Kaiba, and manages to win.
Yugi then uses his powers to put Kaiba under a Penalty Game, in this form, he traps Kaiba in an illusion of the duel monsters world, where Kaiba is continuously eaten by the monsters. This gives Kaiba nightmares from onward, but his resolve is only strengthened. He constructs an entire event at a theme park, in order to plot Yugi's death, as well as his friends. Kaiba kidnaps Yugi's grandfather in order to get Yugi to come, and destroys the Blue-Eyes White Dragon he won from him. In the final battle with him, Kaiba recreates the "shadow game" that Yugi made, with a large glass box that will set of an illusion on the person who loses. Kaiba notes that the shock almost killed Yugi's grandfather. During the set of events that follow, Kaiba nearly kills Honda, as well as his own brother Mokuba.
Kaiba ends up losing the match when Yugi summons Exodia, the rarest, and most difficult to summon card in the whole game. Yugi places Kaiba under another penalty game, this one being Mind Crush, a penalty that ultimately leaves Kaiba himself comatose. But with Dark Yugi later telling Mokuba that his brother is repairing his shattered heart so that the next time he emerges, Seto will not make any mistakes.
From here, Kaiba became more of an anti-hero than a villain, and usually does nothing worse than trying to get in Yugi's way or make fun of Katsuya Jonouchi. In the majority of the ;Duelist Kingdom arc, he was out to save his brother Mokuba, who was kidnapped by Pegasus J. Crawford, but was sealed inside a card after being defeated by him. After Pegasus was defeated and Mokuba, Sugoroku, and himself released, Seto set out to become better than Yugi, and collect all three Egyptian God Cards. He received one, the Giant God Soldier of Obelisk, from Ishizu Ishtar, and created a tournament called Battle City so he could get the other two (making it a rule for the loser to fork over their rarest card to the winner). Here, he was counting on the Ghouls entering (as they possess the two God Cards), however they kidnapped Mokuba (though he escaped) and forced Yugi and Jonouchi to duel each other to the death, but they both survived. He showed his ruthless side again when he was more concerned about seeing the Egyptian Gods in action than about the health of the duelists, which was several times in life-threatening danger.
After the quarterfinals, the tournament was interrupted (IN THE ANIME ONLY) when Kaiba, Yugi, Jonouchi, Anzu, Honda, Shizuka, and Ryuji Otogi entered the virtual world to face Noa and the Big Five, with Kaiba defeating Kogoro Daimon, and later his evil stepfather Gozaburo, who perished when the virtual world was destroyed.
Kaiba lost to Yugi in the semifinals, and with the duel, the Giant God Soldier Obelisk. After Yugi defeated Marik in the final round, Kaiba, as a sore loser, blew up the duel tower in an attempt to kill Yugi and the others, escaping with Mokuba aboard his private jet leaving the others (including Isono) on the island with a damaged ship, but they managed to escape the explosion. However, he was forced to fight alongside Yugi again when Dartz attempted to take everyone's souls for the Orichalcos. He defeated Dartz's henchman Amelda in a duel, causing Amelda's soul to be taken. He and Dark Yugi dueled together against Dartz. Kaiba lost the duel and his soul, but Yugi defeated Dartz, releasing Kaiba's soul from the Orichalcos.
From here, Kaiba never did anything even remotely evil. He hosted a tournament in which he did not enter, and in which the winner would face Yugi in the final round. His enemy, Siegfried von Schroider, attempted to use his younger brother Leonhart to erase all of Kaiba's data. But luckily, Kaiba had it all saved on backup, and this was foiled. Kaiba later entered the RPG to help Yugi fight against Zorc Necrophades, realizing for the first time that everything unusual (such as Hell) was real and not just a trick (as he had thought for most of the show). He witnessed Yugi's duel against Dark Yugi, and finally came to accept that Yugi is the king of games.