Masanori Sasaki, whose true name is Mo Murder, is an assassin who operates under the cover of an ordinary businessman. He is a combat-type synthetic human created by the Towa Organization, one of a small group of artificial beings designed for killing. His outward appearance is unremarkable, allowing him to blend into everyday society while carrying out covert missions. Despite his mundane disguise, his primary function is assassination, and he carries out his orders with clinical efficiency. His motivations are tied directly to his role within the Towa Organization; he acts as an instrument of the group will, eliminating targets without personal hesitation or visible emotion.

In the story, Sasaki plays a crucial role in the events preceding the main timeline of Boogiepop and Others. He is the agent who kills Shinpei Kuroda, a Towa operative who had betrayed the organization by stealing experimental drugs to save Nagi Kirima. This killing is witnessed by a young Touka Miyashita, and the psychological trauma from that encounter contributes to the emergence of her Boogiepop persona. Later, after Sasaki himself is killed, his body is used to frame Makiko Kisugi for her series of murders, allowing the true nature of her crimes to be obscured by the organization. His death thus becomes a narrative device that shifts suspicion and complicates the investigation into the serial killings.

Sasaki's key relationships are defined by his professional connections within the Towa Organization. He is a subordinate who answers to higher-ranking figures, and he interacts with targets like Shinpei Kuroda as an instrument of elimination. He has no known personal bonds or emotional attachments; his existence is purely functional. His development is minimal as a character, as he appears primarily as an obstacle or catalyst for other characters rather than undergoing an arc of his own.

His notable abilities stem from his nature as a synthetic human. He can release tiny vibration waves from his hands, a power that enhances his combat capability and allows him to dispatch targets with precision. He is physically superior to an ordinary human and is trained in assassination techniques. Overall, Masanori Sasaki, or Mo Murder, is a shadowy figure whose actions set major plot elements in motion, yet he himself remains a cold, almost mechanical presence within the world of Boogiepop.