The demon responsible for brokering the original deal with Daigo Kagemitsu that allowed 48 demons to ravage the body of his son, Hyakkimaru. This crafty demon resembles a rat-like creature, although is rarely seen by others. Boasting the power to animate the dead, the monster was able to bolster its image to a more human-like form along with its brethren after taking a part of Hyakkimaru. In the case of the Ratto Oni, it was the child's heart.

Long after the original deal was made, the creature continued to manipulate Daigo Kagemitsu, egging the man toward killing his own son, Hyakkimaru. Once this plan appeared to fail, the demon animated the body of his fallen son, Tahomaru. It offered the cruel warlord a new proposition: give up his own body in return for Tahomaru to be given back his life. Daigo agreed, leading the rat-like monster to burst from the ground and to consume the warlord. Taking on Daigo's appearance, the monster leapt off while relating how he would behead all of those in his way. The terror was short lived, though, as Daigo's human side mustered enough strength to drive his sword through his own body. This act didn't kill the creature, as the demon was far too resilient, but did weaken it enough for Hyakkimaru to deliver a critical blow and vanquish the monstrosity.