Shinya was a student at one of the Order of Imperial Demons' many kindergartens throughout Japan. He was then selected, alongside several talented children, to be the potential mate for Mahiru. This was a plan by the Hīragi to keep their bloodline strong. He was never allowed to go home ever again. His childhood days were filled with training and fighting. Failure to distinguish himself among others meant death. Finally, he was singled out from the other candidates and was adopted into the family and engaged to Mahiru, with the rest of the candidates killed.
It was uncertain whether he bore any true affection towards Mahiru, though he was somewhat jealous of Guren. Despite that, he was depicted to be a true friend towards Guren. He initially initiated a collaborative effort with Guren to crush the Hīragis with him acting as a double agent. Due to the series of events with greater things at stake, that did not come to pass. Over time, he grew to respect and admire Guren and accepted the Kiju (Demon-Curse) from Kureto to save Guren in Volume 4, when Guren was possessed by Noya.
It is revealed by Ferid that eight years prior the story in the manga, Shinya is killed along with Mito Jujo, Norito Goshi, Shigure Yukimi, and Sayuri Hanayori. Their deaths devastated Guren so much that he revived them using the "Seraph of the End", which did revive them, but triggered the apocalypse.