Shimon launches a violent rebellion against the leadership of the Fuma Clan. Exploiting the clan's vulnerability after the Sacred Sword War and during the unexplained absence of key figures Kojirou and Ryoma, he forms the breakaway "Shinsei Fuma Clan" (New Fuma Clan). His core drive is seizing total control of the clan by eliminating its existing leadership and establishing himself as supreme leader.

He wields potent psychic abilities, specifically a powerful form of mind control. This technique mentally enslaves fellow Fuma members, compelling them to betray comrades and fight for his cause. Notable warriors like Muma and Raien fall victim to his control. To ensure absolute obedience and prevent betrayal, Shimon implants a deadly subconscious "Keyword" command within his thralls. This command triggers automatic self-destruction or suicide if the controlled individual attempts to reveal Shimon's identity or plans, effectively silencing potential traitors.

Shimon exploits the clan's weakness and his mind control to orchestrate widespread internal conflict, pitting former allies against each other. His actions cause significant casualties, including major character deaths. Initially operating from the shadows, Shimon rarely confronts opponents directly. He strategically uses manipulated clan members like Raien to lure loyalists such as Shoryu into traps, and relies on controlled enforcers like Muma to eliminate threats. Muma himself is killed by the Keyword command during a psychic battle with Ryoma when he nearly exposes Shimon.

Shimon's ultimate goal is confronting Fuma Clan leader Sousui to overthrow him directly and claim leadership. His rebellion forces the return of Kojirou and Ryoma to defend their clan. Serving as the final antagonist in this conflict, Shimon is ultimately defeated and killed in combat by Kojirou. Following Shimon's death, Kojirou assumes leadership of the reunited Fuma Clan.