Munenori Manyū is the primary antagonist throughout all official media. He holds the position of lord within the Manyū clan and rules Japan, wielding absolute political and social control. Munenori created and ruthlessly enforces a totalitarian system where a woman's social worth, status, wealth, and opportunities depend solely on her breast size. He prioritizes maintaining this breast-centric hierarchy above all else.

He is the father of the protagonist, Chifusa Manyū. When Chifusa rebels against the clan's ideology, steals the secret scroll containing breast-manipulation techniques like the "Breast Flow" (Chichi-nagare) sword art, and flees, Munenori formally issues her death warrant. This act demonstrates his prioritization of clan authority and societal control over family. He directs the clan's considerable resources, including skilled assassins and warriors such as Ouka Sayama and Kagefusa Manyū, to pursue, capture, or eliminate Chifusa and retrieve the scroll. His motivation is preserving the established order that secures the Manyū clan's power.

Physically, Munenori is a tall man with white hair, a beard, and a stern countenance. His personality is cold, vengeful, and authoritarian, showing no remorse. He remains steadfastly committed to the breast-based caste system, viewing Chifusa's quest to redistribute breast size and create equality as a direct threat to his rule and the clan's dominance. His power structure relies on the Manyū clan's specialized warriors, who use unique sword techniques capable of shrinking or absorbing breast size to enforce societal norms and eliminate threats.