Fukaku is the founder of the Myōō Dharani sect who once defeated the Impure King. He is also Ryuji Suguro's ancestor.
Fukaku appears in artwork as a formal priest in a long robe. However, it appears in actuality, he was a relatively young man wearing a standard monk's attire when he battled the Impure King.
Little is known about Fukaku's personality or views. It does however appear that he possessed a certain amount of foresight and caution, establishing the the Myōō Dharani in order to ensure the continued containment of the Impure King, and a system in which the secrets of the sect would be held in confidence through its hierarchy.
Some one hundred fifty years ago, Fukaku battled the Impure King, but couldn't harm it. After forming a contract with his familiar, Karura, he defeated the Impure King, splitting its heart in half. The heart, now referred to as the Left and Right Eye, were sealed away and Fukaku established his own order of exorcists in order to keep it secure.
Upon his death, his contract with Karura was passed onto his descendant.
Fukaku has a familiar called Karura, which takes the form of a small fire bird.