Mayumi Igarashi
The only civilian who knew Jiban's secret identity. She lost her memory in battle, after falling from a waterfall, because, Biolon put in her finger a Bomb Ring. If Jiban approaches her, the bomb will burst. She called Naoto her 'big brother,' only to find that she was really his lost sister in the finale.
Shun'ichi and Shizue Igarashi
Mayumi's (adoptive) parents. Shunichi is the son of Doctor Kenzou Igarashi but is not a scientist. Relocated as witnesses to Biolon, they resumed their life as Mayumi's foster parents in the finale.
Doctor Kenzou Igarashi
Started the Jiban's project to counter Biolon, the results of his biological experimentation. He died when Jiban was built.
Youko Katagiri
A female detective of Central City. Naoto's senior. Manly sharpshooter who could not figure out that Naoto was Jiban.
Kiyoshirou Muramatsu
Elite detective of Central City with a liking for Youko.
Seiichi Yanagida
The head of the Police Department. He cooperated with Doctor Igarashi in the Jiban Project two years earlier. He collaborated with Jiban after Igarashi's death. He died when Jiban's base was destroyed.
Section Chief Takeko Bandou
The Central City Section Chief of Detectives. Got headaches trying to keep the peace in jurisdictions where strange events occurred, no thanks to Biolon.
Michiyo Matsumoto
A tea server of the Detectives' Section of Central City Police Department. She is always cheerful and encouraging.
Boy/Harry Boy
Jiban's robotic assistant. He stays at his secret base. Originally he was simply Boy, an immobile camera-like robot who had to give instructions to Mayumi to operate the control center, but then his computer was transferred to panda-like "Harry" (a robot devised by another student of Doctor Igarashi's) and began substituting for the amnesiac Mayumi. He died when Jiban's base was destroyed.
Ryou Hayakawa
A drifter who takes in Mayumi after she loses her memory, mistaking her for his dead sister, Midori. He initially refers to her by that name but later refers to her by her own name Mayumi after reading a missing person's notice, although Mayumi does not tell him her true origins (since her memory is faulty). In the scuffle, Mayumi goes away, unable to remember what just happened. He died in Episode 46, when Jiban removed the Bomb Ring from Mayumi's finger with a very precise shot, without hurting her. Ryou took the ring and ran, to throw it out, but was killed in the resulting explosion.