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 possessed 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.
 Detective Conan
 Kendo Tournament Murder Case
In contrast to his prior demonic characterization, in the Detective Conan manga canon Takeshi Onimaru appears as an honorable, polite, and serious young man, albeit frightfully massive and skilled at Kendo. He apologizes to Ran when she runs headlong into his hard chest protector and winds up on the floor. He does not help her up though, and seems more concerned with refusing an honorary citizenship (which Momiji set up for him so he would leave the tournament so Heiji could win) he feels he does not deserve due to his a perceived lack of worthiness. Overall, his character appears to more purely embody a Makoto-type samurai mentality who likes fighting strong opponents.
 Swords and techniques
- Fuujin-ken (風神剣 Wind God Sword)
- Maou-ken (魔王剣 Demon King Sword)