Mitsunari Ishida serves as a key retainer to Hideyoshi Toyotomi after Japan's unification. He emerges as a central antagonist by forming a symbiotic relationship with Claudius, a leader of the Genma Triumvirate seeking to resurrect their god Fortinbras. This pact initially grants Mitsunari enhanced abilities, but Claudius progressively seizes full control of his body, using him to advance the Genma's agenda.

As Claudius's host, Mitsunari oversees the transport of Genma Trees—organic constructs fashioned from human bodies—to Kyoto. These trees spawn Genma Insects that infect and control humans, cementing Hideyoshi’s alliance with the Genma. Mitsunari commands brainwashed subordinates, including the warrior Sakon Shima, whom he manipulates via Genma Insects. Confronted by Soki’s group at Fushimi Castle, Mitsunari/Claudius suffers defeat in battle. Though appearing to retreat, Sakon forcibly purges Claudius’s influence from Mitsunari, remaining loyal despite the possession. This leaves Mitsunari unconscious but alive, placed under Sakon’s care to recover.

Mitsunari’s design incorporates a centipede motif reflecting his link to Claudius. His armor features segmented plates resembling an insect exoskeleton, and his helmet evokes a centipede’s head. When Claudius manifests fully, he transforms into a colossal flying centipede wielding scythe-like pincers. Historically, Mitsunari led the Western Army against Tokugawa Ieyasu after Hideyoshi’s death, culminating in the Battle of Sekigahara. While records state he was executed following defeat, rumors persist that Ieyasu secretly spared him.