Seto Kaiba's younger brother, Mokuba first appears in the manga as a spoiled brat who rules at Capsule Monsters. Knowing that Yugi forced Seto to experience a "Penalty Game" after he beat him in Duel Monsters, Mokuba challenges Yugi to Capsule Monsters as revenge. Mokuba cheats, Yugi wins, and Mokuba is then punished with a Penalty Game.
A few issues later, Mokuba invites Yugi and Jonouchi to the Kaiba mansion, where it is revealed that Seto is the CEO of KaibaCorp, making Mokuba the vice president. Once again, Mokuba challenges Yugi to another game; a roulette dinner table that they must take turns spinning and eat whatever dish lands in front of them-- hoping to avoid poison. Yugi outsmarts Mokuba again, and wins.
The next day, Seto Kaiba invites Yugi and his friends to Kaiba Land, a virtual-game amusement park he has created. Seto introduces his virtual reality "duel boxes" (which become the holographic duel arenas in the anime) and duels Yugi's grandfather. The experience of life-size monster holograms shocks Gramps' system and he needs medical attention. Yugi and his friends are then put through "Death-T," an obstacle course of various deadly games, one of them being a rematch with Mokuba at Capsule Monsters.
After Yugi defeats Seto in another game of Duel Monsters, Yami Yugi punishes Seto with a penalty game, and shatters his heart, expelling the darkness from him. Seto is left in a coma-like state as his soul pieces together the remains of his heart.
When asked why Seto created "Death-T" and if it was solely for revenge, Mokuba tells Yugi and his friends the Kaiba brothers' backstory:
When Seto and Mokuba were kids, their parents died and they were left at an orphanage. Mokuba states that life there wasn't all bad, as Seto taught him chess and they played every day. When the CEO of KaibaCorp, Gozaburo Kaiba, came to the orphanage to adopt an heir, Seto challenged him to a game of chess, and if Seto won, Gozaburo had to adopt both him and Mokuba. Seto did win, and the brothers became Kaibas.
But Gozaburo turned out to be abusive, and forced Seto to study constantly in order to prepare him to run KaibaCorp. Seto and the Big 5 (KaibaCorp's board members) bought the company right out from under Gozaburo's nose, and Gozaburo was so humiliated, he threw himself out of a skyscraper window and plummeted to his death. This instilled in Seto the notion that "To lose means to die." That is why Seto can't stand that Yugi beat him.
In Duelist Kingdom, Mokuba was kidnapped and brought to the island because he had swallowed the key to a safe which contained important documents that Pegasus and the Big 5 needed to take over KaibaCorp. He challenged Yugi to a duel, then stole his starchips, and when he was discovered, he explained to Yugi and his friends that he will die to protect KaibaCorp as long as his brother was still in a coma. He was eventually imprisoned in the castle dungeon until Pegasus was defeated.