Yugi's eternal rival and president of KaibaCorp - the world's leading gaming conglomerate. Extremely proud and confident, Kaiba will stop at nothing to become the greatest Duelist in history. Kaiba's signature monster is the legendary "Blue-Eyes White Dragon".
When Yu-Gi-Oh! Duel Links is started after it is successfully downloaded in its entirety, Kaiba will appear confronting Yami Yugi, declaring that when he (Kaiba) defeats him, Yugi will have to call him the King of Games. He then wonders aloud if Yugi even has the cards to confront him. Depending on the player's choice, either he or Yami Yugi can then be utilized in the subsequent tutorial Duel.
After the player concludes their initial in-game tutorial, Kaiba makes an appearance, broadcasting a global announcement across the world of Duel Links. He declares that the Duelists must have realized where they've arrived by now, before revealing that the Duelists present are all here in order to test their skills in the next evolution of Dueling. He proudly states that this world is the perfect place to determine who the real champion is, before rhetorically asking who will wear the crown as Duel World King. Kaiba then reveals that Duel World is divided into different Stages, and he prompts Duelists to reach the final stage to "claim the throne," defeating all who stand in the way of their dreams. Concluding his announcement, he invites Duelists to come to Duel World for their greatest challenge, to test themselves to their breaking points, and dares them individually to become the Duel World King. He then disappears in laughter.
At this point, if the player had chosen Kaiba for their tutorial Duel, he will immediately reappear in Duel World; otherwise, he will only make his next appearance after the player reaches Stage 15. In either case after his appearance, he will declare that everything is proceeding as planned, since the Duelists he invited will help complete Duel World with every Duel they engage in. He then reminisces of Yugi, declaring that both he and Yugi will have their final Duel in this world. (Kaiba's dialogue ends at this point, unless the player had chosen Kaiba as their initial character, in which case Kaiba additionally declares that Yugi will fall against his trusted ally: "Blue-Eyes White Dragon".)
If the player chooses Yami Yugi as their initial character in Yu-Gi-Oh! Duel Links, they will also encounter the following instances of unique dialogue, as they attempt to unlock Seto Kaiba as a playable character.
Until the player has already defeated Kaiba at least once in a non-tutorial Duel, whenever they challenge Kaiba to a Duel, Kaiba declares that only the one Duelist who crushes all foes and snatches victory will earn the title of Duel World King. He then states that the player will unfortunately not be that Duelist, since there can only be one winner: himself. He concludes his dialogue by prompting the player to Duel him.
When Kaiba loses to the player for the first time (in a non-tutorial Duel), he growls before admitting that since only the victor has the right to speak, he'll remain quiet in the meantime. He then reminds the player that this is only the start of their journey to become the Duel World King, before bursting into arrogant laughter.
After the player redeems their final unclaimed reward for Seto Kaiba's Character Unlock Missions, they will encounter Kaiba again, who explains that since his path to the future was clouded and unclear, he created the world of Duel Links as an arena for all Duelists in order to illuminate that path. He goes on to declare that he will rule this world as the King of Games when he defeats Yugi, and that "Blue-Eyes White Dragon" will guarantee his victory, before bursting into prideful laughter.
When starting a Duel with Yami Yugi, he says either "I will defeat you once and for all!" or "Every Duel I've fought has led up to THIS moment! The moment I finally take down Yugi Muto, and regain my top Dueling status! Let the epic battle begin!"
When he wins the Duel, he says "Now YOU will call ME the King of Games!"
When he loses the Duel, he says "Yugi, I was willing to risk everything to win. But I still couldn't pull it off..."
When starting a Duel with Joey Wheeler, he says "I'll show this sniveling puppy that he's more bark than bite!"
When he wins the Duel, he says "Out-Dueling Joey Wheeler is no cause for celebration. I could've beaten him blind-folded."
When he loses the Duel, he says "Heh. So this is what it feels like being the worst Duelist in the world."
When Kaiba Summons "Blue-Eyes Ultimate Dragon", a cut-in frame of Kaiba appears, and he announces "Witness the incredible creature of unsurpassed power! Blue-Eyes Ultimate Dragon!" If that player hasn't already Summoned "Blue-Eyes Ultimate Dragon" during that Duel, a cutscene of "Blue-Eyes Ultimate Dragon" being Summoned plays afterward.
Most of the time when Kaiba declares an attack with that monster, he announces "Prepare for annihilation! Blue-Eyes Ultimate Dragon, attack! Neutron Blast Attack!"
When Kaiba Summons "Blue-Eyes White Dragon", a cut-in frame of Kaiba appears, and he announces "Not a single card can stand up to this!" followed by "I call on Blue-Eyes White Dragon!" If that player hasn't already Summoned "Blue-Eyes White Dragon" during that Duel, a cutscene of "Blue-Eyes White Dragon" being Summoned plays afterwards.
Most of the time when Kaiba declares an attack with that monster, he announces "Blue-Eyes! Lay waste to your enemy! White Lightning!"
When Kaiba Special Summons "Vampire Lord", if that player hasn't already Summoned "Vampire Lord" during that Duel, a cutscene of "Vampire Lord" being Summoned plays afterward.
Oddly, Kaiba doesn't make an announcement about the card before the animation plays.
When Kaiba Summons "XYZ-Dragon Cannon", a cut-in frame of Kaiba briefly appears, and he announces "I special summon XYZ-Dragon Cannon!"
Most of the time when Kaiba declares an attack with that monster, he announces "XYZ-Dragon Cannon, attack! Unleash your firepower! Cannon Firestorm!"
When Kaiba activates the effect of that monster, he announces "XYZ-Dragon Cannon's effect activates! Hyper Destruction!"
When Kaiba Summons "Kaibaman", he announces "It's like looking in the mirror! Here comes Kaibaman!"
When Kaiba declares an attack with "Kaiser Glider", he announces "Go Kaiser Glider!" followed by "Golden Burst!"
When Kaiba Summons "Vorse Raider", he announces "I Summon Vorse Raider!"
When Kaiba Summons "XY-Dragon Cannon", he announces "I Special Summon XY-Dragon Cannon!"
When Kaiba Summons "XZ-Tank Cannon", he announces "I Special Summon XZ-Tank Cannon!"
When Kaiba Summons "YZ-Tank Dragon", he announces "I Special Summon YZ-Tank Dragon!"
When Kaiba activates "Enemy Controller", he announces "I activate Enemy Controller" followed by "Enter the code! ←→AB!"
When Kaiba activates "Megamorph", he announces "The Spell Card, Megamorph!"
When Kaiba activates "Monster Reborn",he announces"I activate my Spell Card! Monster Reborn!"
When Kaiba activates "Polymerization", he announces "If you didn't already realize how much better I am than you, you'll find out now! I activate Polymerization!"'
When Kaiba activates "Ring of Defense", he announces "I activate my Quick-Play Spell, Ring of Defense!"
When Kaiba activates "Soul Exchange", he announces "I activate the Spell Card, Soul Exchange!"