Nishinosuke Akai is a plain-clothes police sergeant serving under the South Edo Magistrate in the mid-nineteenth-century city of Edo. He is a low-ranking officer with a rigid obsession with order and cleanliness, exhibiting what can be described as obsessive-compulsive tendencies that compel him to constantly clean dirt and trash from the streets. This fixation extends to his personal dislike for the residents of the Furai Row-House Block, whom he views as disorderly and contemptible; he harbors a particular hatred for Ginjirou, one of the main inhabitants. His primary motivation as a policeman is to enforce the strict moral laws of the era, but his role often places him in direct opposition to the protagonists, especially when their illicit activities—such as building fireworks—threaten his sense of law and order.

As the story progresses, Akai’s character undergoes a dark and significant transformation. He becomes entangled with a blue sky beast named Yu, with whom he develops a sexual relationship. To sustain her, he resorts to murder, feeding her blood. His loyalty eventually shifts from the magistrate to Yu, and he agrees to help her find a way back to the moon, even before the alien girl Sora receives assistance. This development reveals a deeply conflicted personality: initially a petty, antagonistic figure obsessed with cleanliness and authority, he later becomes a desperate and morally compromised individual driven by a twisted form of attachment.

Notable abilities of Nishinosuke Akai are less about exceptional physical skills and more about his dogged persistence and his knowledge of the city’s underbelly as a policeman. He is capable of ambushing and confronting others, as seen when he tests Ginjirou’s combat abilities with the help of the Men in Black. His key relationships are defined by animosity and later by obsessive devotion: his hatred for the row-house residents, especially Ginjirou, and his morbid bond with Yu. Throughout the series, he evolves from a strict enforcer of law into a figure willing to break every rule for his own dark purposes, marking a stark trajectory from antagonist to a tragic, villainous participant in the larger narrative.