Numabuchi Kiichirou, who grew up in the Gunma Prefecture, was a serial hold-up murderer who got involved with the Black Organization. He was originally hired to become one of their assassins, but his mental liability soon exempted him from that career. Instead, he was to become a human guinea pig for the new APTX 4869 poison, under the supervision of Miyano Shiho. However, while Shiho had been given his file for close medical study, she didn't get to meet Numabuchi personally; knowing about his impending fate, he escaped the Black Organization. During his flight, he panickedly murdered several people he suspected of being Black Organization pursuers.

While he may be cast as a psychotic serial killer, in the series Numabuchi appears more like a frightened victim. While he may kill without hesitation, he does it mostly out of fear and to regain his freedom. He is shown to have a more sensitive side and a fondness for children when he rescues Mitsuhiko from an ill-fated attempt to present his crushes Ayumi and Haibara with a special surprise.