Katō is a supernatural Melmoth the Wanderer-style figure with a mysterious past. He is described as an oni born from the grudge of 2000 years of Japan's hidden history.
However Katō is also a powerful sorcerer; a skilled onmyoji who manipulates oni. Later on in the novel, his birthplace is determined to be Ryūjin, Wakayama and associations are made between his lineage and Abe no Seimei's clan. The implication is that Kato's ancestors were heretics who practiced onmyodo magic outside the established government.
In the 19th century, he enlists in the Imperial Japanese Army under the guise of a Japanese soldier and rises to the rank of First Lieutenant.