Sōsō commands a punitive military force that invades the secluded Juuza cat village under the guise of pursuing Yellow Scarves rebels. His campaign forcibly displaces the cat tribe from their homeland, igniting the central conflict. During the clash with the Yellow Scarves, he kills their leader—exposed as a Juuza—and manipulates Kan'u into desecrating the corpse to shield her tribe. This calculated act frames the Juuza as scapegoats, escalating human persecution against them. He subsequently captures Ryuubi, the cat tribe's young leader, holding him hostage to coerce the Juuza into staying within human territory and eliminating remaining rebels.

Despite employing ruthless strategies, Sōsō displays unexpected nuance by confining Ryuubi comfortably with toys and sweets. He also engineers a coup against the anti-Juuza governor Toutaku, temporarily allying with the cat tribe for mutual benefit—a move that brands them as traitors. His military prowess manifests in large-scale operations, notably a siege where he directs 100,000 troops against a heavily outmatched foe. After witnessing Kan'u's combat skill, he attempts to recruit her, though she refuses. Their ensuing duel sees Sōsō gain advantage, yet he spares her upon learning his stronghold is besieged, choosing strategic withdrawal over personal triumph.

Sōsō's fixation on Kan'u intensifies upon discovering her half-human heritage, deeming her a vital asset. This obsession fuels his pursuit through multiple engagements, including a failed attempt to seize her by force during battle, thwarted by Ryuubi's intervention. Historically noted as a celebrated ruler, he is renowned for ambitions of unification, excelling in warfare, governance, and cultural arts like poetry.