Byoki

The creature known as Byoki began his life as a seemingly simple house cat during the late Heian Period. At this time his name was Haimaru and he belonged to Sana, the daughter of a powerful onmyodo master. Skittish, Haimaru recoiled at the touch of anyone save for his mistress who carried him in her arms almost everywhere she went. The only other person he seemed to mildly tolerate was Mao was was able to pet the cat when no others could.

Eventually Haimaru was abducted by Mokuzu, a simple-minded groundskeeper who was promised great power if he would aid in a nefarious plot to disrupt the school. Mokuzu stole the cat and dumped it into a pit filled with venomous creatures and wild animals. Amazingly the cat was the sole survivor but was radically changed after devouring the poisons and blood of its vanquished foes. It has evolved into the power cat demon Byoki. Breaking free of its bonds Byoki devoured the sacred texts of the onmyodo temple, gaining knowledge and then attacking the practioners. Originally having been intended to be used as a weapon against other schools, Byoki turned against his masters and murdered indescriminantly.

Finally he faced off against Mao who managed to decapitate him. Byoki tried to merge himself with Mao to gain more power but failed. Instead Mao merged with Byoki's severed body while his head fled, going into hiding beneath a sacred stone until the Great Kanto Earthquake of 1923 liberated him. It was then that the beast fought Mao again, spilling his blood into a time lost Nanoka and passing through the time portal into the present day.

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The Meaning Behind the Name - 猫鬼

Byoki is written (猫鬼) which would typically be read as "Nekomata" (cat demon). However the alternative reading is used for the prounciation of the creature's name. Byoki is also a homonym for "illness" when written (病気). The cat's original name, Haimaru (灰丸), is written with the kanji for "ash" and "circle" (maru is a common suffix in masculine names).