Kojiro Soma lives in Tokyo as a high school student with his younger brother Sanshirou and their dog Patra. His routine existence changes when Akito Kobayashi transfers to his school; the two share a past from elementary school where Kojiro defended Akito from bullies and initiated him into occult practices, including demon summoning. This childhood connection resurfaces as Akito expresses lingering emotional attachment, hinting at deeper feelings for Kojiro.

Initially unaware of his hidden supernatural lineage, Kojiro's life fractures after a demon attack orchestrated by Akito results in Patra's death. A priest intervenes, sealing the demon Cerberus within Patra's body to protect Kojiro. Soon after, Sanshirou is pulled into a demonic computer program, prompting Kojiro and Cerberus to follow. Inside this digital dimension, they face Gagyson, a demon draining Kojiro's life energy (magnetite).

Kojiro's true nature is unveiled as the reincarnation of Masakado Taira, the guardian deity of Kanto. Kyouko Shibusawa performs a ritual to unlock his divine power, an act leading to her excommunication from her order. Awakened as Masakado, Kojiro battles Ose (called Satan in some versions), a powerful demon summoned by Gagyson using Saki Yagami's life force. Overwhelmed by Ose's regeneration, the conflict reaches its climax when Akito sacrifices himself to resurrect Saki as the goddess Konohana Sakuya. This act enables Kojiro to defeat Ose. Konohana Sakuya then pacifies Masakado's spirit, and Akito expresses gratitude to Kojiro before dying.

In other media, Sanshirou Soma appears independently in *Giten Megami Tensei: Tokyo Revelation*, searching for Kyoko Shibusawa after losing an arm in an unspecified conflict, though Kojiro himself does not feature.