Mokuba Kaiba is the younger brother of Seto Kaiba and serves as the vice president of KaibaCorp. During the fifth season of Yu-Gi-Oh! Duel Monsters, which encompasses the Kaiba Grand Championship arc and the Pharaoh's Memories arc, Mokuba remains a constant presence at his brother's side. He is characterized by his unwavering loyalty and deep devotion to Seto, whom he loves and supports unconditionally. Mokuba is notably more good-natured and naive than his older brother, and he tends to trust Yugi Mutou and his friends more readily than he did in earlier seasons. Despite his youthful age, he demonstrates maturity beyond his years, often acting as a voice of reason when Seto's pride or ruthless ambition threatens to cloud his judgment. Mokuba is not afraid to question Seto when he believes his brother is making a mistake, showing that his loyalty does not prevent him from thinking independently.

Mokuba's primary motivation is to see his brother smile again, as he remembers a time before Seto became the serious and ruthless head of KaibaCorp when they could cheer each other up during their difficult childhood at an orphanage. He is also driven by a desire to protect his brother and the company, even placing himself in danger to ensure Seto's success. In season five, Mokuba plays a supporting role during the Kaiba Grand Championship, the tournament Seto hosts to showcase KaibaCorp's technology. He aids in the tournament's operations and remains alert to the sabotage orchestrated by Zigfried von Schroder, a former rival of Seto who seeks revenge. During the Pharaoh's Memories arc, Mokuba continues to support his brother as they become involved in the events surrounding the ancient Egyptian past and the battle against Yami Bakura.

Mokuba's key relationship is with his brother Seto Kaiba. He is the only person to whom Seto consistently shows love and affection, and their bond is the emotional anchor for both characters. Mokuba recognizes the good in Seto that others often overlook, and his presence brings out his brother's more human and positive traits. He also develops a cordial and trusting relationship with Yugi and his friends, a notable shift from earlier seasons where he was more cautious of them. Mokuba's intelligence and technical knowledge make him a valuable asset; he is adept with advanced technology and can provide strategic support during duels. His role is not that of a primary duelist, but he occasionally duels and displays competence, using decks that often feature plant-type monsters. Overall, Mokuba Kaiba in season five is a mature, loyal, and perceptive young boy who serves as his brother's closest confidant, moral compass, and most trusted ally.