Originally a very serious person, Onimaru could not get along with Yaiba Kurogane at all and the two quickly became rivals. In order to defeat his hated enemy, Onimaru began training with his family's sacred Fuujin-ken until he became possesed by it and became an actual demon (the character for oni in his name means demon). He soon took over all of Japan, traveling in a flying castle. His minions include Kumo-Otoko and the Hakki, the Kuroki, the Shiteno, and many completely black oni. Eventually, he switches from the Fuujin-ken to the Maou-ken.
After Yaiba cuts off his horns he returns to being a normal human. At first he viewed Yaiba as a nuisance, but after training with Yaiba's father Kenjuro, he returns for the final story and now acknowledges Yaiba as a friendly rival. During the Oda Nobunaga Tournament he has a dove for a pet.
 Swords and techniques
- Fuujin-ken (風神剣 Wind God Sword)
- Maou-ken (魔王剣 Demon King Sword)