Kiyotsugu Kamiazuma holds a significant position within the Kamiazuma clan. He is the father of Haruhiko Kamiazuma and the uncle of Yumiko Kamiazuma. His brother, Kiyoharu Kamiazuma, descended into madness and forcibly implanted a shard of a magical mask into Kiyotsugu's eye. This shard carries Kiyoharu's memories, causing Kiyotsugu to periodically believe he is Kiyoharu.

When influenced by the shard and operating as Kiyoharu, Kiyotsugu mistakes Yumiko for his sister Yuriko, Kiyoharu's actual sister and Yumiko's mother. In this state, he subjects Yumiko to the same pattern of sexual abuse that Kiyoharu inflicted on Yuriko during her childhood and adolescence, which resulted in Yumiko's birth. Outside these episodes, Kiyotsugu engages in an incestuous relationship with Yumiko as himself.

His actions contribute directly to Yumiko's profound mental and emotional scars, affecting her psychological stability and dependence on the goddess Juna for normal functioning. Kiyotsugu's role involves perpetuating cycles of abuse rooted in the Kamiazuma family's history. The cursed mask shard ensures Kiyoharu's pathological behaviors and memories persist through Kiyotsugu, making him an unwilling vessel for his brother's legacy of violence and incest.