One of the 48 demons who pillaged the body of Hyakkimaru and was therefore granted the ability to take on a human form, the Maimai'Onba was in reality a butterfly-like monster. Its center was that of a pale woman with long white hair, yet its arms were giant wings flanked with countless eyes. With the ability to change into a person, though, the demon decided to mate and take shelter with a human man. Their union resulted in the Maimai'Onba giving birth to a hive of children that could change from human to insect as they desired. However, they still hungered for the flesh of other people, and their parents set out to satisfy this hunger.

Starting up an orphanage out of a temple, the Maimai'Onba posed as a nun who took in stray children either abandoned by their parents or whose family was killed by war. Once the children were boarded inside, the demon and her offspring fed on their flesh, leaving only their bones and clothes behind. The plan worked up until the temple was struck by lightning and subsequently burned to the ground. The parents had to change their targets to stray outsiders instead, as the demon spied on people in her small butterfly form while her husband drew them to their house.

On one such occasion, the pair brought in Hyakkimaru and Dororo to their residence. Unknowing to them, Hyakkimaru was a trained demon killer, and made short work of their children when they came to feed. Furious, the Maimai'Onba persuaded her husband to attack Hyakkimaru, but ended up running her own sword through the body of both Hyakkimaru and the man she claimed to love in an effort to kill them both. However, Hyakkimaru's wound quickly healed as the butterfly-like demon tried to escape. Taking flight, the monster was thwarted by the efforts of the spirits contained in the Huge Fetus, which were the dead children her own offspring had fed on. The apparitions dragged the demon back down to earth where Hyakkimaru slashed it, ending its reign.