Shien Shinano serves as the elder brother to Seiran and Seiju Shinano and uncle to Mayuki Hyuga. He resides permanently in Switzerland, tasked with protecting Shara Hyuga, Mayuki's mother. Shien temporarily returns to Japan to check on Seiran and ensure Mayuki's safety, assuming the headmaster role at Mayuki's elementary school during this period.

His personality is notably childish and irritating, frequently provoking exasperation in others despite his responsibilities. Yoko, sister to Aya and Byakko, acts as his secretary and disciplines him with a harisen when his behavior becomes overly bothersome. Like his brothers, Shien commands a puppet warrior—Yoko herself—as a doll user.

Seiran initially harbored hatred toward Shien, though their relationship evolved into mutual understanding over time. Shien suffers a severe physical defeat during a confrontation with Seiju, revealing his limitations. His presence in Japan indirectly spurs Seiran to greater initiative against Seiju's schemes.

Shien's core motivations center on familial protection and duty, focusing on safeguarding Shara in Switzerland and ensuring Mayuki's safety. His return to Japan catalyzes heightened engagement in the conflict against Seiju.