Mushin is an elderly Buddhist monk who serves as the guardian and mentor of the monk Miroku. After the Wind Tunnel curse consumed Miroku's father, Mushin took in the orphaned boy and raised him within a secluded temple located at the site of the crater left by that tragedy. This temple becomes a recurring sanctuary for Miroku throughout his journeys. Mushin is characterized by a complex personality that blends a fondness for drinking and a somewhat gruff, lazy demeanor with genuine wisdom, affection, and a deep sense of responsibility toward his pupil. Despite often appearing drunk or dismissive, he is knowledgeable about demonic threats and spiritual practices, and he possesses the practical skill to repair Miroku's Wind Tunnel when it is damaged.

In the second film, The Castle Beyond the Looking Glass, Mushin plays a pivotal supporting role. Following the apparent defeat of the villain Naraku, Miroku returns to the temple. Mushin then entrusts him with an important mission: to destroy a powerful yokai that threatens to plunge the world into eternal night. This directive directly sets the film's main conflict into motion, as the yokai in question is later revealed to be Kaguya. Through this task, Mushin acts as a catalyst for the group's next adventure, reinforcing his role as a figure of guidance who passes on essential knowledge and responsibilities to the next generation. His relationship with Miroku is that of a father figure and teacher; he raised him from childhood, taught him the ways of a Buddhist monk, and continues to watch over him from a distance, offering advice and occasional repairs to his cursed hand. Mushin does not undergo significant personal development within the film, but his presence underscores Miroku's own growth and the legacy of the monk's family. His notable abilities include his spiritual knowledge as a Buddhist priest, his capacity to mend the Wind Tunnel, and his familiarity with ancient demonic legends, which allows him to provide crucial context for the threat the heroes face.