Mokuba is Seto Kaiba's younger brother and only blood relative. Their parents were both dead by the time Mokuba was 3. They were sent to an orphange where Kaiba told Mokuba he would take care of them and never trust anyone but themselves.

Kaiba and Mokuba are really close, they care for each other as much as brothers should. But it was not always like this, their competetion in games drove them apart. After his turn in the Death-T against Yugi, his lost angered his brother who was willing to punish his only blood.

When Kaiba was defeated by Yugi, and was sent into a coma-like state, Mokuba traveled to the Duelist Kingdom planning on defeating Pegasus to restore his beloved. When he met up with Yugi and his friends Kaiba was restored, but Pegasus sealed Mokuba's soul into a Duel Monsters card, same goes with Kaiba. When they were both revived by Yugi, Kaiba realized how much he and his brother love each other.