Muu functions as the enigmatic pitch-black crow familiar to Beast Knight Mikazuki Shinonome, consistently portrayed with minimal visual detail. She maintains near-total silence throughout the narrative, speaking only during their initial contractual pact formation and immediately before the familiars' departure at the story's conclusion. This pervasive quiet renders her a near-invisible background presence despite constant proximity.

A pivotal revelation in the final battle between Yuuhi Amamiya and Mikazuki unexpectedly identifies Muu as female, startling both the Beast Knights and fellow familiars. This disclosure establishes her as the sole female familiar within the group.

In combat, Muu provides essential domain control abilities to Mikazuki. During the concluding events, they jointly receive Hresvelgr's power, enabling a balanced confrontation against Yuuhi and his familiar. Their partnership remains operationally effective across all shared battles despite minimal verbal interaction.