The Grand Master is the elder leader of the Beast Kingdom, a parallel world inhabited by anthropomorphic creatures. He is a figure of immense wisdom and authority, appearing as an old rabbit. His most significant act is the decision to retire from his position, which for a being of his stature is not simply a resignation but a spiritual transformation, as he plans to reincarnate as a deity. This decision sets the entire plot of The Boy and The Beast into motion, as it creates a vacancy for a new leader.

The Grand Master’s personality is defined by his impartiality and his insightful, almost testing, nature. He does not simply choose the most obvious successor. Instead, he names two potential candidates: the popular and honorable Iōzen, and the powerful but lazy and isolated Kumatetsu. This choice reveals that he values hidden potential and inner truth over mere reputation or decorum. While Iōzen is the respectable and expected choice, the Grand Master shows a distinct affinity for the brash Kumatetsu, sensing something genuine in him that lacks the dark secrets he suspects in his rival. Rather than declaring an immediate victor, he creates a condition for the succession duel, suggesting that Kumatetsu must find a disciple to inspire him and make him a worthy contender. This action demonstrates his role not just as a ruler, but as a mentor who engineers a scenario for growth.

The Grand Master’s primary motivation is to ensure the future stability and strength of the Beast Kingdom by choosing the right successor. His methods are indirect but effective. He understands that Kumatetsu, for all his raw power, needs to overcome his loneliness and laziness to become a true leader. By pushing him to take a disciple, the Grand Master inadvertently orchestrates the meeting between Kumatetsu and the human boy Ren, who becomes his student, Kyūta. In this way, his single decision to retire and his subsequent guidance shape the central relationship of the entire story.

In terms of his role, the Grand Master is the catalyst for the film’s conflict and the arbiter of its resolution. He presides over the succession duel between Kumatetsu and Iōzen, and his declaration of Kumatetsu as the winner is what ultimately leads to the revelation of Iōzen’s dark secret concerning his human son, Ichirōhiko. His judgment is final and absolute. The Grand Master has a pivotal relationship with Kumatetsu, acting as a former master to the unruly beast. There is a sense that the Grand Master has a deep, almost paternal understanding of Kumatetsu, seeing the potential in him that others do not. He also connects, though from a distance, to the human world through the presence of Ren and the troubled Ichirōhiko, recognizing the dangers of the darkness that can grow in a lost heart.

The Grand Master himself does not undergo personal development, as he is a completed, static figure of wisdom. His development is external, reflected in the successful growth of his chosen successor. However, his final act is the most dramatic transformation. After Kumatetsu wins the duel, the Grand Master fulfills his plan and reincarnates, becoming a deity. In this new form, he ceases to be a character and becomes a precedent, demonstrating the ultimate destiny for a ruler of the Beast Kingdom.

The Grand Master's notable abilities are less about combat and more about spiritual and political power. He has the ability to reincarnate as a god upon his retirement, a privilege exclusive to his position. He also possesses the authority to name his successors and oversee the ritualistic duel that determines the next lord. While not a fighter in the main narrative, his very presence commands respect, and his judgment is shown to be profoundly far-sighted, setting into motion events that ultimately save both the human and beast worlds from a destructive force.