Cozzakill, I am American. Onimaru means (Ogre)Demon King.
The Japanese don't have a word that equals Demon. Demon comes from the Greek word 'D(ae)mon' meaning enlightened spirit. The Catholic church labeled all things pagan as evil so d(ae)mons become the monstrous denizens of hell that come to here to corrupt people. Over time the (ae) character was converted to being an 'e' and the connotation of it being a monster was made permanent.
The Japanese describe 'Oni' as an Ogre, a real monster that lives on earth. However, they are also revered as magical beings. This magical origin is the reason Western Culture made Oni synonymous with Demon. Even in anime they translate it to demon.
I do not speak any other languages than English. I do, however, have a powerful need to learn and research things and I am willing to cite my references upon request. If you ask super nice I will even do so in any English standard format. (APA, MLA, etc.)
Thank you for asking