The older half of the Arson Brothers, Zenmei gained his reputation by burning down a police station with his younger brother, Muroto Koumei. An extremely dangerous man, Zenmei has had a history in juvenile prison, which was where he met the Naitou Twins (Naitou Ichigo and Naitou Ichie), both of whom he took under his wing.
Hisashi Amachi gains Zenmei's respect through a confrontation between the two and after realizing that Amachi is no longer the spineless brat he once knew, Zenmei resolved to follow him into the Amachi Army. During his stay with the faction, he also recruits the deadly Naitou Twins to assist him in assignments Amachi would give him. His sheer size and bulk alone are enough to intimidate opponents, making him a truly fearsome guy to mess with.
He commands the pseudo-shadow force of the Army, as he and the Naitou Twins are often seen operating independent of the faction's main body. Regardless of his "separation" from the Army however, Zenmei and the Twins still take their orders from Amachi. He challenged Hanaki Guriko to a fight during the Army's war against the Hana-Gumi, but lost.
He no longer sticks around with the faction once they are soundly defeated, but entrusts the twins under the care of his younger brother, Koumei. Zenmei has also turned over a new leaf and the twins mention that he is currently trying his best to leave his delinquent days behind.