Mitsuhiro Maniwa is a junior police detective assigned to the investigation of a series of street assaults carried out by a mysterious figure known as Shonen Bat, or Lil' Slugger. He serves as the younger, more even-tempered partner to the brusque and aging detective Keiichi Ikari. In both appearance and demeanor, Maniwa is neatly groomed, gentle, soft-spoken, and open-minded, often acting as the calming counterpart to his partner's short fuse. He approaches witnesses and suspects with kindness and patience, consistently seeking to understand the emotional undercurrents of the case rather than relying solely on concrete evidence.

Maniwa's core motivation stems from a genuine desire to uncover the truth behind the attacks, even if that truth is elusive or uncomfortable. He is the first to notice a pattern among the victims: each person attacked by Shonen Bat was under extreme psychological pressure, backed into an emotional corner. This insight leads him to theorize that the assailant is not merely a criminal but a manifestation of collective paranoia and repressed stress. His willingness to follow intuition and consider abstract possibilities often frustrates his pragmatic partner, who prefers hard facts.

As the investigation deepens, Maniwa's empathetic nature becomes both a strength and a vulnerability. He bonds with the troubled boy Makoto Kozuka, who later becomes an imitator of Shonen Bat, and he is able to perceive the link between the attacks and the delusional fantasies of the victims. Most significantly, he pieces together the dark past of Tsukiko Sagi, the character designer whose childhood trauma gave rise to the cartoon mascot Maromi, and realizes that Maromi and Shonen Bat are two sides of the same psychological phenomenon.

Maniwa's immersion in the case gradually erodes his own mental stability. He becomes obsessed, neglecting his health until he appears gaunt and hollow-eyed. His grip on reality frays, and he begins to hallucinate, notably seeing himself as the superhero Radar Man in his own reflection. He physically confronts Shonen Bat using an umbrella, which he perceives as a sword, but these efforts are futile. By the end of the series, Maniwa is left a shadow of his former self, psychologically broken by the very delusions he sought to understand. His partner Ikari, who resists fantasy, ultimately succeeds in resolving the crisis, while Maniwa's insight into the nature of paranoia comes at the cost of his own sanity. His journey illustrates the danger of becoming too deeply entangled in the fantasies one tries to dissect.