Mataichi commands the Ongyou, a trio of supernatural investigators operating in Japan's Bakumatsu era who pursue legends and deliver justice to sinners. He adopts the guise of a spiritualist or monk, typically shrouded in bandages and a cloak. His core purpose is identifying individuals tainted by grave crimes, especially murderers radiating a detectable "dark aura," and engineering confrontations where they face their misdeeds before execution.

Utilizing psychological manipulation and supernatural powers, Mataichi provokes targets into confession or breakdown, often coordinating with his companions Ogin and Nagamimi. Following a forced confession, he enacts retributive justice upon the sinner, marking the body with an ofuda to commit their soul to the next world. He concludes each ritual with the phrase "ongyō shitate matsuru."

His methods embody a rigid moral code focused solely on punishing wrongdoing, frequently involving psychological torment and culminating in death. Though generally dismissive of the idealistic writer Momosuke, Mataichi permits his presence during investigations and occasionally cautions him about the dangers of associating with the Ongyou. He initially operates under the directives of Kyogoku Tei, who supplies targets, but shifts allegiance upon uncovering Kyogoku Tei's scheme to plunge Japan into darkness. Mataichi ultimately participates in the final conflict that defeats Kyogoku Tei, leading to his own implied demise.

Mataichi possesses abilities including supernatural perception and ritualistic exorcism, and consistently leads the Ongyou unit. His origins, personal history, and development across other media remain undocumented, and his role within the series shows no significant evolution or expanded backstory.