Mû is a supporting character from the anime Lucifer and the Biscuit Hammer, serving as the crow-type beast attendant. As a familiar, Mû is bound to the Crow Knight, Mikazuki Shinonome, and exists as part of the group of animal attendants who support the twelve Beast Knights in their mission to protect the princess and fight against the mage Animus. Mû is a being of few words; the character is consistently depicted as taciturn and rarely speaks, communicating only when necessary and often in a minimal or silent manner. This reserved nature gives Mû an air of mystery and observance, standing in contrast to the more vocal and expressive personalities among the other attendants and knights.

In terms of background, Mû is one of the ancient, cyclical attendants who have participated in the repeated battles between the princess's forces and the mage over the course of history. Like the other beast attendants, Mû was created or summoned to serve a specific knight and help guide them in the use of their abilities. Mû's primary role is to support Mikazuki Shinonome, a college student and skilled martial artist who becomes the Crow Knight. Mû interacts with Mikazuki as his partner and guide, providing counsel during both combat and moments of decision, though their bond is understated due to Mû's quiet demeanor.

Motivations for Mû are aligned with the broader duty of the attendants: to see the Beast Knights succeed in their mission so that the world can be saved from the Biscuit Hammer and the machinations of Animus. As a long-lived being who has witnessed previous cycles, Mû likely possesses a deep understanding of the conflict's stakes, though this perspective is expressed sparingly. Mû's personal desires are not elaborated upon in available sources, with the character remaining primarily defined by the role of a loyal and stoic attendant.

Mû's notable abilities are not explicitly detailed, but as a crow familiar, Mû likely possesses enhanced perception, the ability to fly, and the capacity to communicate with or influence the knight's powers. Beast attendants in the series are tied to the knights' magical abilities and often serve as conduits for the activation of the knights' unique combat techniques. Mû's taciturn nature and sharp observational skills complement Mikazuki's more impulsive and battle-hungry personality, creating a dynamic where Mû's quiet presence serves as a grounding force.

Development for Mû is limited within the narrative, as the character remains a consistent background presence rather than undergoing personal change. Mû's relationship with Mikazuki is the most significant, with the two working as a pair throughout the conflict. Mû also interacts with other attendants and knights, but these exchanges are brief and often nonverbal. The character's enduring trait is a calm, watchful stillness that persists even in the midst of battle.