Sotetsu Ibaragi is the playwright for the traveling Yuyama Troupe kabuki group, addressed as "Sensei" by its members as a mark of deep respect. He possesses notable intelligence and maintains an air of mystery around his true motives. Sotetsu joined the troupe after its founding by leader Kakunojo Yuyama, her father's acquaintance Zagashira, and apprentice Ebisu, but before other members like Shiranui Kozo and Kakashi no Keishin were recruited. While outwardly supporting the troupe's mission of revenge against Hario Genba for murdering Kakunojo's parents, Sotetsu secretly pursues his own goal: obtaining the supernatural artifact known as the "Head of the Conqueror" (Hasha no Kubi).

His methods involve manipulating historical events from behind the scenes. Early in the story, he engages in undisclosed business with the merchant Jubei Nakaiya (also known as Kuroiwa Sennosuke), who is connected to the auction of the Head of the Conqueror in Yokohama. Later, Sotetsu allies with the historical figure Takeaki Enomoto, leveraging Enomoto's efforts to rebuild a republic during the Bakumatsu period's political upheaval for his own undisclosed plans.

Sotetsu wields significant influence. He secured Shiranui Kozo's membership by crafting a play that saved him from execution. He also arranged the troupe's performance at the Sekkakurō pleasure house in Yokohama through his connection with the high-ranking courtesan Kotoha. His actions position him as a key, enigmatic figure navigating the era's turbulent political and supernatural conflicts.