Shinoa, like her half-sister,Mahiru hīrgai, was conceived through artificial insemination by using the seeds of Tenri Hīragi in various demon-possessed women. The demon Shinoa was born with was shikma Dõjin somewhere in her early childhood, Mahiru somehow separated the demon from Shinoa in order to save her younger half-sister which causes Shinoa to forget about Shikama Dōji.

As a child, Shinoa was overshadowed by her older half-sister for being a genius and fighter prodigy that her father was looking for; hence, Shinoa was never acknowledged by her father along with most of her family and even lived alone in her own apartment. The only person she had was Mahiru, who often visited and treated her nicely to the point that Shinoa was willing to be her older sister's messenger until Mahiru eventually abandons her. Shinoa is eventually reunited with Shikama Dōji which causes her to regain her memories of him and gain the demon as her Cursed Gear.

While Mahiru is considered a hero for developing the Cursed Gears that the JIDA now relies on today, Shinoa ended up getting shunned by the Hīragi family largely thanks to Mahiru's betrayal in siding with the brotherhood starting the war between them and the Imperial Demon Army, and becoming possessed by the demon curse that lead to her death.

At some point, Shinoa is appointed sergeant and has been working under Guren.