Mokuba Kaiba is the younger brother to Seto Kaiba and is one of the supporting characters in the anime/manga Yu-Gi-Oh!
In the Yu-Gi-Oh! anime, Mokuba is Seto Kaiba's younger brother, and just wants his brother to be happy. He believes that Yugi killed who he was after beating him, and wanted to beat him as revenge. In the end, Yugi convinces him that he had helped him, and Mokuba joins them in the duelest kingdom.
Mokuba takes a much different attitude in the manga, as a mean little kid, who cheats whenever his oppenent is too good. He wants his brother to aknowledge his ability, though Seto only makes him feel weak. He appears after the first time Yugi beat Seto, and challenges him to Capsule Monsters, a game which he believes he is the best at. Though he rigs the game from the start so that his monsters would be more powerful, Yugi still comes through in the end. Mokuba challenges Yugi a second time, right before his final battle with Seto. He cheats again, and once again Yugi beats him despite that.