Mokuba Kaiba is the younger brother of Seto Kaiba and appears in Yu-Gi-Oh! The Movie: Pyramid of Light as a supporting character. He is a loyal and affectionate sibling who consistently supports his older brother, even as Seto grows cold and ruthless after taking over KaibaCorp. Mokuba deeply cares for Seto and wishes to see him smile again, a warmth he remembers from their time together at an orphanage before their adoption. Despite his devotion, Mokuba is not afraid to question Seto when he believes his brother is making a mistake, showing a strong moral compass and independent thinking.

In the events of the movie, Mokuba accompanies Seto to the Duel Dome where Seto forces Yami Yugi into a duel. When the duel takes a dangerous turn and the building begins to crumble, Mokuba flees the structure to safety. Later, he is rescued along with Tea Gardner and Solomon Muto by Maximillion Pegasus, who arrives in a helicopter. Mokuba's presence throughout the story serves as a grounding emotional anchor for Seto, who will risk everything to protect his brother. At the film's conclusion, an injured Seto departs with Mokuba, reiterating his vow to defeat Yugi another day.

Mokuba's personality is defined by forgiveness and resilience. He bears no grudges and is quick to forgive, especially his brother, despite the hardships he endures from being frequently kidnapped by Seto's enemies. He holds a position as the vice president of KaibaCorp, indicating his involvement in the family business and his competence beyond his age. While not a duelist in the movie, his role underscores the theme of brotherly bonds and provides a key emotional counterbalance to Seto's obsessive rivalry with Yugi.