Mokuba is Seto's younger brother. Mokuba cares about his brother more than anyone else in the world. Vise-versa, Seto feels the same way towards Mokuba. Both of them will risk live and death just to be together, and they share a remarkably strong brotherly bond.
When they were young and at an orphanage, Seto was chosen by many people for his genius, but refused everyone that wouldn't take Mokuba aswell. After they were adopted, it would seem that their new step-father only wanted Seto's genius and skill for his own.
Seto spends almost all of his time with machines. Mokuba likes to spend time with his brother and therefore is one of the few people who can work lots of Kaiba's gadgets. However, unlike his brother, Mokuba feels a strong sense of friendship towards Yugi and his friends.
