Shinjuro Yuki's origins stem from childhood tragedy when he lost his parents young. Raised by various caretakers who instilled a strong ethic of helping others, this philosophy propelled him into war-torn regions. There, he delivered electronic appliances and films to offer hope and distraction to afflicted communities. On these missions, he encountered the Japanese volunteer group "Singing on the Battlefield," comprising Yuuko Kurata, Tatsuya Yamaga, Myoushin Oono, Yasuo Saburi, and Kumi Yasuda, guided by his former high school classmate Makiro Serada.

A critical turning point unfolded as Shinjuro's group entered a conflict zone. An attack caused their vehicle to crash inside a tunnel. While evacuating everyone before the vehicle exploded, Shinjuro suffered a fatal neck injury from a stone stake. His blood revived Inga, a nearby sealed supernatural entity. Inga possessed Shinjuro's body, healing his wound, but proceeded to consume the souls of several group members by forcing them to reveal painful truths. Yuuko Kurata committed suicide to prevent Inga accessing her soul, leading Inga to inhabit her body temporarily. Regaining control, Shinjuro struck a pact with Inga: she would refrain from killing when consuming souls, and in exchange, he would provide opportunities for her to feed on human "screams"—truths revealed under duress—through his detective work.

After these events, Shinjuro was framed for Myoushin Oono's murder through manipulations by Serada and the cult leader Bettenou. To evade arrest, he adopted a new identity provided by Izumi Koyama—Shinjuro Yuki—once his original identity was publicly discredited as a murderer. This marked the start of his detective career.

Professionally, Shinjuro operates as a detective in post-war Tokyo, frequently collaborating with the Public Prosecutors Office on sensitive cases. He employs meticulous deduction but occasionally utilizes Inga's ability to compel individuals to answer one question truthfully. His insistence on uncovering factual truths often clashes with government and corporate interests, particularly those of media magnate Rinroku Kaishou, who manipulates case outcomes to maintain public stability. This dynamic earned Shinjuro the derisive public nickname "The Defeated Detective," though law enforcement circles privately acknowledge his skills.

His partnership evolved after rescuing Kazamori Sasa, a Real Artificial Intelligence (RAI) targeted for disposal. Kazamori integrated into his team, assisting with digital investigations and providing analytical support, supplementing Inga's more confrontational methods.

Shinjuro exhibits stoicism and occasional irritability, yet demonstrates consistent compassion, forming protective relationships with allies like Inga and Kazamori. He negotiates unconventional agreements to aid others, reflecting his foundational belief in helping people. His war zone experiences contribute to underlying trauma, sometimes surfacing in outbursts during failed deductions or confrontations with systemic corruption. A significant development occurred when Bettenou temporarily controlled Inga, forcing Shinjuro to solve cases independently and reaffirm his commitment to truth-seeking without supernatural aid.

Throughout his investigations, Shinjuro remains ideologically opposed to censorship and government cover-ups, positioning him as a counterforce to Kaishou's media manipulations. Despite setbacks where truths are suppressed for political reasons, he persists in exposing realities, driven by his conviction that societal healing requires transparency.