Sōgen, a death-row convict with multicolored hair in distinctive buns, was recruited into a covert operation as a replacement for the slain Shinsengumi captain Keisuke Yamanami. His arrest and death sentence stemmed from conducting illegal dissections on abandoned corpses and developing euthanizing medicines, reflecting his medical and scientific background. He continues applying this expertise within the substitute unit, using analytical methods to investigate phenomena like unexplained magical swords encountered by the group.

He avoids direct physical confrontation, favoring self-made explosives as his primary weaponry during missions. This unconventional tactical preference aligns with his non-combatant specialization and inventive approach. His scientific curiosity drives collaborations with allies such as Suzuran when confronting mysteries that defy conventional understanding, including supernatural weapon analysis.

His alias "The Psycho Doctor" references both his medical background and past involvement in ethically questionable practices prior to joining the operation.