The younger half of the Arson Brothers, Koumei gained his reputation by burning down a police station with his older brother, Muroto Zenmei. He is approached by Hisashi Amachi himself to join the Amachi Army, which is an offer Koumei initially refuses. But when Amachi says that he can clear his father's debt if he decides to join his faction, Koumei agrees and even convinces Zenmei to join in as well.
During his time with the Army, Koumei served as one of the faction's leaders and was the group's main muscle, alongside his older brother. He also became close friends with Kagawa Kazuya (a.k.a. Gaga) upon joining the faction. Koumei is extremely hot-headed and almost any decent conversation with him will be sure to end in a heated confrontation. He prefers using his fists rather than his head and would rather prefer picking fights to strategizing.
His strength as a fighter is well-known, enabling Koumei to take down several key players in the Army's ascent to power. When the Army finally ran into Suzuran's Hana-Gumi, Koumei faced-off against the group's second strongest man, Sakota Takefumi. Completely overwhelmed by Sakota's strength, Koumei ended up losing their fight.
He is currently still a leader of the faction and remains loyal to Amachi to this day. He is now also the leader-figure of the Naitou Twins (Naitou Ichigo and Naitou Ichie) after Zenmei entrusted them under his care. As an active member of the Army, Koumei continues to serve Amachi against The Manji Empire. However, he and his friend Gaga were jumped by the Manji's Kawauchi Group (led by Nakamori Shouji), thus sparking Amachi's anger and eventual participation in the war against them.