Byoki wasn't always the creature he is now. Originally just a house cat named Haimaru during the late Heian Period, he belonged to Sana, the daughter of a powerful onmyodo master. He was skittish—refusing to let anyone touch him except Sana, who carried him everywhere. Mao was the one rare exception, managing to pet him when nobody else could get near.
Everything changed when a groundskeeper named Mokuzu stole Haimaru and threw him into a pit full of venomous creatures and wild animals, hoping to gain power by helping orchestrate some kind of attack on the school. Somehow the cat survived alone in that pit, but not unchanged. After devouring the poison and blood of everything he'd killed, Haimaru transformed into Byoki—a powerful demon cat.
Freed and hungry for more, Byoki consumed the sacred texts of the onmyodo temple to gain knowledge, then turned on the very practitioners who'd created him, killing indiscriminately. He was supposed to be a weapon against rival schools, but he had other plans. When Mao finally caught up with him, she managed to decapitate him—but Byoki tried to merge with her for a power boost. That didn't work out. Instead, Mao merged with his body while his head went into hiding under a sacred stone.
He stayed trapped there for centuries until the Great Kanto Earthquake of 1923 shook him loose. Byoki fought Mao again, and this time his blood spilled into a time-displaced Nanoka, sending him through a portal straight into the present day.
Content compiled by AnimeList.moe from publicly available sources.
