Shuin mentors and guards protagonist Inyo, having raised and rigorously trained him in lacht—a power allowing materialization through understanding an object's mechanics and composition. Their teacher-student dynamic features Shuin's harsh tutelage, including deliberately withholding the true nature of Inyo's pet creature Toto. This omission triggered Toto's violent rampage and death, an event that deeply scarred Inyo. As enduring mementos, Shuin crafted distinctive cat-shaped pins resembling Toto, which Inyo wears on his hat.

Before his unexplained disappearance, Shuin tasked Inyo with protecting a girl named Zero should they meet, though his connection to her remains undefined. Since Shuin vanished, Inyo maintains their former Magmell residence, returning yearly to clean it while hoping for his mentor's return.

Physically, Shuin possesses white-grey hair and matching grey eyes. He typically dons a bronze-brown hat and cape-like coat over a black shirt and pants, mirroring Inyo's style. His demeanor radiates calm command, yet beneath this reserved exterior lies genuine care for Inyo's well-being.