Ki'ichiro Numabuchi, 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 lability 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 Shiho Miyano. 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.
In his younger years (before he became a murderer), Numabuchi and his fellow classmates from driving school caused the death of their stern driving instructor Inaba. Years later, his son, Yusuke Sakata, who had become a police officer to investigate his father's death, caught Numabuchi, who blabbered out a confession. Since his father's death case was long since time-barred, Sakata decided to murder all those responsible, keeping Numabuchi for last in order to put the blame on him. However, Conan and Heiji cracked the case, and Numabuchi was taken into custody.
Several months later, Numabuchi was escorted into a forest in Gunma after having claimed that the body of a fourth victim was buried there. Actually, he intended to pay a last visit to his special childhood friends, the fireflies, living in the forest before his execution. There he met Mitsuhiko, who was attempting to collect some fireflies in order to rejoice his friends Haibara and Ayumi. Instead of doing him any harm, however, Numabuchi carried Mitsuhiko back to the search parties and had himself arrested without putting up any resistance.