Kayama is the primary target of investigation in the film Zero Woman Returns. He is a dealer in illegal firearms and narcotics, specifically known for buying contraband that has been confiscated by corrupt police officers within the system. This places him at the center of a network of dirty cops and underworld commerce.

The most defining characteristic of Kayama is that he is a man without a past. Official government records contain no information about him, making him effectively invisible to the state. He appears in no official documents or databases, a trait he shares with the film's protagonist, Rei, the special agent of the covert Section Zero. Kayama is described as the perfect soul mate for Rei precisely because of this shared non-existence, as both operate in a shadow world hidden from the average citizen.

Within the story, Kayama serves as the mission asset for Rei, who is assigned to investigate and bust his criminal operation. However, the situation becomes complicated as the investigation progresses. Kayama turns out not to be the person that Rei’s superiors initially believed him to be. Rei finds herself in a complex position because, in addition to this revelation about his identity, she becomes attracted to him. This attraction, combined with their mirrored status as individuals erased from official existence, creates a unique and dangerous bond between agent and target. Kayama's presence in the narrative forces a convergence of professional duty and personal connection, challenging the usual black-and-white dynamic of an operative and her mark.