Muneyuki Manyū holds the high-ranking position of chief breast inspector for the Manyū clan, a role achieved despite his lack of swordsmanship talent. His duty involves evaluating women's breasts to enforce the clan's societal hierarchy, where status is dictated by breast size. Years of constant exposure and analysis have fostered within him a profound aversion to large breasts, making his duties a source of personal torment.

To clan members, Muneyuki appears composed and dutiful. Secretly, however, he harbors ambitions to capture his sister Chifusa. His goal is to extract the "Chichi-nagare" breast-manipulating technique from her, intending to master it himself. He believes this mastery will end his suffering, masking this motive behind a facade of clan loyalty.

Following Chifusa's defection, Muneyuki demonstrates ruthlessness in preserving the clan's rule. He aligns completely with his father Munenori's brutal methods, issuing death warrants and employing lethal force against dissenters, revealing darker traits that contrast with his earlier respected image.

His relationship with his formidable swordswoman bodyguard, Ouka Sayama, is complex. He values her exceptional combat skills over her flat chest—a rarity in their society—and trusts her with his secret ambitions. While Ouka's romantic feelings for him are implied, Muneyuki's reciprocal emotions remain ambiguous, though he consistently relies on her unwavering loyalty for missions targeting Chifusa.